I suppose I couldn't be more wrong about those micro Four-Thirds lenses, could I? The fanatics seem to think that there are no better lenses.
Since I got the Olympus MMF-3 adapter, I'm pleased to say that the Olympus SHG ZD 14-35mm f/2.0 works very well with the GH3. The lens is very heavy for a bit more than a 2x zoom but at the wider angles, you need more work to get a great lens. That's why you see so few of them. I'm leaving the adapter attached to the 14-35mm, while I can use the ZD 35-100mm f/2.0 with the Olympus E-5 and it's better balanced as a combination.
Notably, there is a problem if you insist on auto focus but it's similar to the problem on Four-Thirds bodies. The 14-35mm lens will hunt far too much, similar to the 50mm macro. Sadly, this is the revised lens that arrived in 2007 with the Olympus E-3 body and not the original lens that was supposed to be available in 2005. That was apparently an optical masterpiece but somehow flawed in that it had to be redesigned. The current, available 14-35mm lens is still an optical masterpiece but extends on both ends of the zoom range.
In any case, the 14-35mm can auto focus with the GH3 through the adapter, and it's faster than the experience with the Olympus E-1. I often use manual focus so I'm sure about the details. Focus assist/pinpoint focus is useful but as I'm turning the ring, it can be a bit bouncy. Carry Dramamine, if necessary. Since manual focus eliminates the automated hunting, I can get precise focus more quickly.
The 35-100mm f/2.0 is nearly perfect and the combination is a bit unbalanced, a bit more than it was with the E-1 but it works quite well, once again, it can be auto focused by the GH3 more quickly than with my E-1, but it's much faster with my E-5.
The optical quality of the two lenses is extremely high and I'm quite pleased with the results with the GH3. Even in dim light, the combinations work much better than with the E-5. I'm looking forward to buying and using the 7-14mm f/4.0 SHG lens. If you're wondering why I wouldn't buy the much less expensive, much smaller Panasonic version, it's very simple: weather-resistance.
Why, oh, why would you spend nearly US$1000 for a lens if some driving rain or dust storm could affect the insides? That said, I have the excellent Leica/Panasonic 25mm f/1.4 for Four-Thirds. I'm concerned about taking it outside and really using it, unless it's a perfectly clear day or night. It focuses quite quickly with the GH3, as well as the E-5.
I'm surprised to have made the (half a) leap to micro Four-Thirds, although the MMF-3 adapter is my very good friend. For now, the Olympus SHG zooms continue to rule my world, with the HG zooms a close second and I've had the 14-54mm f/2.8-3.5 and 50-200mm f/2.8-3.5 since 2004. Further behind would be the premium Panasonic 12-35mm f/2.8 (which I don't have) and 35-100mm f/2.8 lenses.
Update 2013.09.16: Obviously, the same lenses will work wonders with the newly announced E-M1. Of course, with the somewhat useful Phase Detection Auto Focus, they'll even work similarly to the E-5 but at a slower pace, and I suspect will work poorly in lower light, but that's not been shown that I can see. The GH3 can auto focus to -4 EV, which is like starlight--something the opposite of intense. There is no such specification that I can find for the E-M1, so maybe Olympus are re-working the firmware already to improve the performance of the auto focus.
Update 2014.02.11: The other day I had to laugh. Someone had posted a comment on an article about the soon to be released Panasonic GH4, calling it an utter failure because with Nikon, Canon, and Sony mirror-less system cameras you could mount dSLR lenses, and you couldn't do that with FujiFilm (you can't but the official adapter supports Leica M-mount) or micro Four-Thirds.
Given that Olympus produces some of the finest dSLR lenses in their High Grade and Super High Grade lines, I can't imagine how deep ignorance goes. I'm still not happy with micro Four-Thirds lens selections although Olympus has upped the game with the 12-40mm f/2.8 lens and their intended 40-150mm f/2.8 lens that has not arrived yet. I'm getting ready to send off my Panasonic 35-100mm f/2.8 lens because of huge lens flare but it hasn't been totally bad or good. It's just not very good for the price--Olympus HG line was a steal in comparison.
Update 2014.02.25: The other day, I bought the Olympus 12-40mm f/2.8 micro Four-Thirds lens for my Panasonic GH3. I am surprised at how much better it makes the GH3 photos look, as compared to the Panasonic 35-100mm f/2.8 lens, even though it was less expensive. You'd think Panasonic would work incredibly well with Panasonic. I sent that lens off to be repaired, due to a rather huge problem with lens flare, something I saw documented about the time I bought mine, but didn't expect that there would be a problem. Getting a repair seems to be a problem in the U.S.A.
In any case, by the end of 2014, Olympus will have the 12-40mm f/2.8 and 40-150mm f/2.8 lenses available. Some time in 2015, they'll also have 7-14mm f/2.8 and a 300mm f/4.0 lenses. I haven't seen anything about a 150-300mm f/2.8 lens but it needs to be part of the plan.
While the 12-40mm f/2.8 isn't as great as the 14-35mm f/2.0, it is great for the price, and appropriately positioned between the SHG 14-35mm on the high end and the HG 14-54mm f/2.8-3.5 at the middle position, above the standard kit lens. I'm also pleased that they've finally addressed the 12mm focal length. I know it's more difficult to control the distortion, but it's worth having. The new for 2015 7-14mm f/2.8 will be great for landscapes and also for indoor real estate, which too often is left to a point-and-shoot camera.
Update 2014.03.25: I'd like to add that all of my Four-Thirds lenses work well on the GH3, especially the Leica/Panasonic 25mm f/1.4 that was meant for the Digilux 3. It seems to be meant for Contrast Detect Auto Focus, so it moves quickly. Since the GH3 has a top shutter speed of 1/4000th of a second, the f/1.4 in full sun is close to being overwhelmed, so a Neutral Density filter could be a good idea. The GH4 will fix this problem as it has the 1/8000th shutter speed. How did we manage in the old days with f/1.4 or f/1.2 lenses and 1/1000th of a second shutter speed?
I'm happy to say that my experience with the new Olympus 12-40mm f/2.8 lens, I have found renewed respect for Olympus and I am certain to buy the 40-150mm f/2.8 lens, though I feel a bit ambivalent about micro Four-Thirds still, especially after my ordeal with the 35-100mm f/2.8 lens and finding that it supposedly didn't need repair. If Olympus would produce a somewhat bigger body, like a downsized E-5, or the E-1 with modern innards, I'd be very happy. I'm not yet a hybrid shooter, though I'm going there and the GH3 and GH4 are on the right path, but Olympus does so many things right with the E-M1 that I wish they'd finished it properly.
Update 2014.10.17: I've been shooting with the E-M1 for roughly 4 months, having traded the GH3 before the trade-in value dropped. The 12-40mm has been working well, though I've found one instance where it produced nasty lens flare. It's rare that I've used any of my Four-Thirds lenses on it, save the Leica/Panasonic 25mm f/1.4 because of the E-M1's small size. I'm looking at the 40-150mm f/2.8 very seriously, along with the matching 1.4x teleconverter. The combination is still cheaper than my Olympus 35-100mm f/2.0 lens, and the effective f/4.0 maximum aperture is only a bit slower than my 50-200mm f/2.8-3.5 on the long end where I use it most.
Update 2015.03.11: I've had the Panasonic GH4 since December and it reminded me how good the design of the GH3 was. It's much easier to use my Four-Thirds lenses than it is with the Olympus E-M1.
While I love the 12-40mm f/2.8 as compact as it is, they should have made it a bit bigger and made it uncompromisingly wonderful. The 14-35mm f/2.0 is amazing in ways that the 12-40mm f/2.8 can't be but the lack of those 2mm on the wide end does make it somewhat incapable when taking architectural photos. Olympus had a remedy for this problem with the 7-14mm f/4.0 but using it without artificial light indoors was difficult. Now, they're bringing a 7-14mm f/2.8 to market for micro Four-Thirds, which seems perfect for higher end real estate. I'm planning to use it for skate park photos mainly, along with the 8mm f/1.8 fisheye lens.
micro Four-Thirds has come a long way, and there are a few good lenses now, but Olympus' Four-Thirds lenses are still much better and an easy way to get great image quality.
Update 2015.08.07: Since I have the Panasonic 35-100mm f/2.8, Panasonic/Leica 15mm f/1.7, Olympus 12-40mm f/2.8, and 8mm f/1.8 fisheye lenses, the adapter doesn't get as much use on either my Panasonic GH4 or Olympus E-M1. Occasionally, I will mount the Leica/Panasonic 25mm f/1.4 on it, as it can be difficult to get lower light photos, but then, I'm finding the new Nikon D7200 and Sigma 18-35mm f/1.8 lens more difficult in such conditions, despite their supposed technical superiority. Firmware updates will likely help.
Except for the 35-100mm f/2.8, the micro Four-Thirds lenses are very, very good. They're not as good as my Olympus SHG lenses and on occasion, my HG lenses are more useful, but it's rare for me to use the adapter. The best reason I can see to use it is to mount the Panasonic GM5 to my ZD 35-100mm f/2.0 and watch people be shocked at the appearance.
Update 2016.01.19: With the Olympus 300mm f/4.0 lens, supposedly there is a micro Four-Thirds lens that has exceeded the possible image quality of its Four-Thirds equivalent. I'm not in a position to judge. The micro Four-Thirds lens is about half the price of the Four-Thirds lens. While the maximum aperture isn't as big, the sensors of the Four-Thirds' bodies aren't very good in lower light. In the case of wildlife and bird photography, a thin depth of field is as difficult as it is helpful.