I've been using Four-Thirds equipment since around April 2004 with the E-1, 50-200mm f/2.8-3.5, and the 14-54mm f/2.8-3.5, plus FL-50. At times, it's been a chore to not jump ship.
When I first bought the E-1, I was amazed at the color rendition and clarity. Outside, during the hurricanes of 2004 in Florida, I was impressed with the durability and weather-sealing. I looked at Nikon, Canon, and Pentax prior to buying into Four-Thirds and none of them were very good at the time. Certainly, I would have spent much more money but many things have changed since then.
In 2004, raw development software was almost non-existent, save what the camera maker included. Olympus put out JPEG files that were so far ahead of the competition that it wasn't funny.
Since then, I've changed to the E-5 as my day-to-day camera body with the Panasonic GH3 as a backup, but the E-1 is still there just fine. I've added the 35-100mm f2/.0 and 14-35mm f/2.0 SHG lenses and the 50mm f/2.0 macro, plus the Leica/Panasonic 25mm f/1.4. They work fine on all three bodies.
I'm considering this new Olympus body. There have been some rumored specifications, and they look reasonable, considering the micro Four-Thirds E-P5. There was a "leaked" video, supposedly from Engadget using an Olympus representative of some sort. What I saw was sloppy and led me to believe that it's fake but who knows? For the first two minutes, the Olympus representative referred to Four-Thirds and Four-Third.
While I doubt my hand is as big as his hand, I think that a camera body the size of the E-M5 with a grip reminiscent of the E-1 but sort of welded onto the body in a haphazard way will not work well, except in casual photography. micro Four-Thirds has mostly been about casual photography, except for those few professionals. I believe that the latest sensors are quite good and produce great image quality. but that's not enough when photographing sports.
If I can't get the shot, who cares how great the image quality is. I've tried this multiple times with the GH3 and I want to throw the thing. The EVF stands in the way of getting the shots.
What's the problem? To photograph runners, you need to be able to change the focus quickly and put it on a certain runner. Auto focus works willy-nilly, so it goes mostly where you don't expect it, even in high end cameras from Nikon and Canon---that's why professionals shoot so many shots rapid-fire. In order to use manual focus, which is my preference, you need to keep the assistance to a minimum. Most micro Four-Thirds cameras with generate a magnified view to help you. This gets in the way of what I'm doing, whereas an optical viewfinder just shows the view. Of course, back in the 1970s and 1980s, we had multiple focusing screens to help us.
I suppose what I will see is a huge compromise. A too-light camera body with a lens adapter with good auto focus capabilities, but otherwise, all the problems of a mirror-less camera body. It's not that the E-5 isn't a compromise over the E-1--it is--but it works well for what I do. However, it has terrible image quality as the light dims, even though with studio lighting, it's quite good.
Could they have used the E-1 or E-5 bodies and just replaced the innards? I suspect that they could have. Would the 1.8 inch rear display of the E-1 been a problem for today's video-oriented crowd? Certainly, but just give me a !@#$ stills camera that works almost as an extension of my body!! (Update 2013.11.13: Nikon has done that with the Df body. It's all about still photography, although the price and changing format is not high on my list. The technology is great, the slippery body is not.)
What I haven't mentioned is the cost of manufacturing a single, low quantity body. They showed this when they extended the E-30 production until exhaustion, without an update to an E-50.
What gets to me is how clumsy they've been since the initial E-1. Instead of a replacement in 2005, they produced several consumer-oriented dSLRs, and waited (or couldn't help a delay) until 2007 to produce a professional, 2005-level dSLR. Equally, the E-5 arrived in 2010, but would have been a great 2007 model. People blame the financial fraud, but I doubt that had anything to do with it. I believe that they had people in charge of projects who should have only been making tea.
I understand quite well about projects, deadlines, cost, and mistakes. If you're methodical, you'll do well, but if you aren't, everyone suffers. Regardless, accidents happen. The E-M1 looks like a huge accident, in opposite ways from the E-3, but just as big an accident.
Update 2013.08.24: I've seen a few more photos of the E-M1 and they look as though it's not a rough pre-production unit. It looks quite a bit better but still tiny. The 12-40mm f/2.8 looks very good, but too stylish, as if it's been made to be in photos and not made for photography. I just hope it works well, since the very average Panasonic 12-35mm f/2.8 doesn't appeal to me at US$1200.
Update 2013.11.13: After using both the E-M1 and the 12-40mm f/2.8 lens, I'm happy for Olympus to be pushing itself further, thanks to Panasonic. Competition is a great thing. I like the E-M1 but it doesn't fit my hand well. The 12-40mm f/2.8 is a must have for me. I can only hope that the Panasonic GH3 will be replaced by a GH5 with the same sensor and some greater focusing speed.
Update 2014.05.03: It's been some time since I tried the E-M1 and Panasonic's GH3 replacement has been announced as the GH4, although the GH3 is being continued for a while, as a lower cost body.
The E-M1 has done quite well, and I've considered it time and again. The GH4 doesn't have Phase Detection pixels but it may work as well as the E-M1 when focusing my Four-Thirds lenses. The GH3 generally does well enough, except for the 14-35mm f/2.0 and 50mm f/2.0 and those can both be a handful, even on the E-5 or E-1.
Update 2015.01.10: I've had an E-M1 since June 2014, trading my GH3. It's been good but not great. Around Thanksgiving, I got a GH4, thanks to a US$200 off deal.
I still don't think that the E-M1 is a true successor to the E-5...or E1, but it is what the company decided would be available. It works well enough but I don't see that it's good enough with Four-Thirds lenses. The balance is off, starting with my 50-200mm f/2.8-3.5. Obviously, the 35-100mm f/2.0 is not a good lens for balance with the E-M1--or the GH4. It is good with the E-5 and nearly as good with the E-1.
I still believe that they should have used the E-1 body as the basis for the E-M1. The video capabilities of the E-M1 aren't great, so they should have just given us a stills camera body.
After all this time, I still go back to my E-1 and E-5 when I want to use my Four-Thirds lenses, except for the Leica/Panasonic 25mm f/1.4, which fits nicely on the GH4 and is more than acceptable with the E-M1.
Update 2016.12.14: The E-M1 Mk II is more-or-less available at this time. It is the successor to the E-5, in most every way. I still don't like the grip. I felt that the grip from the E-5 was comfortable, as is the grip on the GH4. The GH4 seemed as the physical successor to the E-5 or E-1 bodies.
The E-M1 Mk II supposedly handles every situation well, although low light situations are never easy. I'd like to see how the auto focus works around 10 in the evening. I don't have trouble with the GH4, but the E-M1 tends to hunt.
Since the Mk II has 121 cross-type sensors, both PDAF and CDAF, it should be just short of phenomenal.