Having used the GH3 extensively, and traded it recently while the trade-in value was still good, and now, having the E-M1, I've been making mental notes about things that work and don't work.
I'm warming up to the E-M1, having taken 2300+ shots on Saturday. It is both a great and annoying camera body.
Auto Focus and Burst Mode
The auto focus is fast. I have found in single auto focus mode that it can almost always keep up. Of course, as the light is diminished, every auto focus system loses. It has surpassed the AF of the GH3, although I have noticed a huge difference in burst mode and/or AF with the Panasonic 35-100mm lens. It works significantly more slowly than the 12-40mm.
This lens is able to be focused very quickly on the GH3 and works at the full 6 fps in burst mode. On the E-M1, something is not working correctly, as there is a noticeable difference in frame rate. Perhaps, the lens is only able to work at 6 fps with OIS off? I'll check again when I get my GH4.
However, it focuses very well with the Leica/Panasonic Four-Thirds 25mm f/1.4--the 62mm filter size lens, not the tiny micro Four-Thirds version. I still have yet to use my other Four-Thirds lenses with it. I should really see how the 50-200mm f/2.8-3.5 performs, as this is my go-to lens for outdoor sports.
Black out with sunglasses
My Oakley sunglasses and the EVF combine to give me black-out conditions in bright sunlight, as with the GH3. I should try my free Jack in the Box sunglasses to see if those change the situation. I've saved a number of photos by simply using muscle memory. I've even got to the point where I don't have the camera to my face at times.
The battery life is horrible. Who thinks a small battery is a good idea with a powerful EVF, and with an uncovered rear display that wants to show you everything much of the time? Of course, it's better if you don't use the rear display at all, and I've found ways to minimize my use of it but the EVF still drains the battery quickly, along with the 5-axis stabilization. It's my mistake to think that the E-M1 will go as long as the GH3, but it seems to end just as the GH3 is warming up.
The grip is still uncomfortable. The GH3 is probably the camera body with the most comfortable grip on the market right now, for me, anyway. It's right up there with the Nikon D800 and the Olympus E-5. I suspect that Olympus wanted to save money by using an existing battery, and designed the grip around it--to be too small. I've heard from a number of people that it's just fine. Maybe, they have fatter fingers than I have. Someone suggested that I buy the battery grip, but it will be difficult to use its bigger grip in landscape orientation.
Accidental control changes are too easy. Coming from the E-1 and E-5, the front and rear dials change aperture and shutter speed, depending on the mode, of course. The front dial of the E-M1 changes exposure compensation, by default. Sure, it's a new user error, but considering that the E-M1 was meant to be the replacement for the E-5, they should have left the functionality as it was (with the E-1, E-3, and E-5), and allowed an optional change for those who wanted it. It's also far too easy to switch the auto focus point. Yes, I've been using a single point because I don't like the typical scatter pattern. I will accidentally tap the OK button (enabling the Super Control Panel) and arrow keys when I'm not shooting, and then, I miss shots, and have to return to reset it, missing more shots. If the rear display was on a fully-articulated panel that could be closed, this wouldn't happen.
Super Control Panel
The Super Control Panel really is a great way to display and change controls. The Quick Menu from Panasonic really doesn't work well for me, and I spend more time trying to figure out how to get to what I want and to change it (not just view it and think that I've changed it)--than to actually change it and get back to shooting on an Olympus body. Does the Olympus menu system suck? YES. Does the Panasonic menu system suck? YES. People claim that Nikon, Canon, Sony, etc. have a better system. They don't. They all suck. It's the Super Control Panel/Quick Menu kind of interface that helps you get work done more quickly when the body lacks physical controls.
This bugs me the most. Why didn't Olympus use a fully-articulated display, as with the E-5? Having used the GH3 to get very low architectural photos with the 12-40mm f/2.8 lens, I can only guess in doing this with the E-M1. The display doesn't flip out. It goes up or down. Having the panel out in the open all the time leads to all sorts of possible dangers. It's also responsible for accidental control changes and reduced battery life.
I have not used my Four-Thirds lenses extensively with the E-M1. I have used the Leica/Panasonic 25mm f/1.4 lens--the one with the 62mm filter size with the E-M1. As with the GH3, it focuses well. From what I understand, it was meant to be compatible with Contrast Detect AF, as the Leica Digilux 3 was designed for Live View, as well as the typical dSLR optical viewfinder experience. Thankfully, for this lens, we're in drought conditions. I would hate to have a downpour and find that a US$1000+ lens is ruined by rain. I have probably used it fewer than 10 times outdoors.
I'd really like to see if the 50-200mm f/2.8-3.5 works well with it, as I use it for cross country meets. Olympus' micro Four-Thirds 40-150mm f/2.8 will become available later this year, but it doesn't reach nearly as far. Sure, I can carry a load and run the course, because...no, I only do that occasionally when I cannot see past the corn fields.
Olympus has done a good job creating a top end model for micro Four-Thirds. It's not what I'd hoped that it could be, to replace my E-5. Yes, the performance is good. No, it's not quite right. Panasonic thought more about what the photographer wanted and put that into the design of the GH3 and GH4. In fact, the GH3 felt like the true successor to the E-5. It's the kind of design where you don't have to think how to use it--you know how to use it. On the other hand, the E-M1 is fiddly, unlike the E-5 or E-1.