I've been an Olympus equipment user since the 1990s. I've gone through various bodies since my initial OM-1N. When Olympus showed that they were ready for dSLRs, I bought an E-1, 50-200mm f/2.8-3.5 and 14-54mm f/2.8-3.5--all weather-resistant equipment, which was good since I was in Florida where the hurricanes, daily summer rains, and the construction dust would damage most equipment easily.
I used the equipment out in three hurricanes and it's still functioning beautifully.
Since then, I bought an E-5 body. The odd, less ergonomic design was bigger, more Nikon-like and made it easier to balance the 35-100mm and other SHG lenses.
I recently, and somewhat temporarily traded my Panasonic GH3 for an Olympus E-M1. I'd been using the GH3 for indoor sports where flash was a serious problem and where lighting was as advanced as the 1950s. It worked reasonably well. The E-M1 is a bit more useful with the 5-axis image stabilization, but is uncomfortable in my hand. They should have created a new, bigger battery and used it to create a bigger, better grip like that of the E-1, E-5, or GH3.
In any case, I've taken some good photos with it, but I had not put it to the test with sports. Between moving cross country and schools being out, I didn't have much of a chance to photograph sports.
The other day, the Lathrop, California Generations Center had its grand opening celebration and I was able to photograph some skaters. I think the E-M1 acquitted itself quite nicely, as you'll see. I still can't wait for my Panasonic GH4 but I must.
Update 2014.07.05: I used the E-M1 with my Four-Thirds Leica 25mm f/1.4--the big, heavy one made for the Leica Digilux 3 body. It worked pretty well with auto focus in low light. Perfect it wasn't, but conditions were far from perfect.
Update 2014.07.25: I've noticed in the last few days that the E-M1 will suddenly not focus properly. I assume that it's still busy writing to the card but that's not always the case. Perhaps, it's cleaning up after itself.
Update 2014.10.29: The other day, I tried the full area (what I call splatter) auto focus pattern and got plenty of photos where nothing was in focus. This seems as good as it was on the E-5, prior to switching to a single point for AF.
I'm planning to try the 5 area AF to see how it works, but I don't have much hope.
Update 2014.12.03: Geez, the 5 area AF made sure that I didn't get any in-focus photos. Everything was crap.
About a week ago, I bought a Panasonic GH4. I've been able to use it in varying conditions, and in darkness around sunset, it wasn't great, as expected. However, indoors, photographing skaters on a half pipe, it was amazingly good, and more responsive than the E-M1 was. I took many video clips, and they all looked surprisingly bright.
Update 2015.02.20: Any moment now, there should be a firmware update, version 3.0, that will increase the tracking speed frames per second rate from 6.5 to 9.0 fps. They apparently came up with a better algorithm to use the 37 phase detect pixels.
Update 2015.03.13: I tried the C-AF + Tracking option and while it works, it seems to lag. It could be the circumstances--artificially-lit skate park during the evening but I suspect it would still be laggy in good light, just better.
Regardless, I find the auto focus, especially with face detection, quite good and better than the GH4 for stills. The GH4 auto focus is quite capable for video. Panasonic really needs to update the firmware for a few small things.
Update 2015.12.19: I'm still amazed how good the single AF is, especially with face detection enabled. I've got a lot of photos--so many that I had to have the shutter replaced. The shutter is rated at 150,000 actuations. At 10 frames per second, you can get there quickly.
I bought a Nikon D7200, mostly for wide angle shots. I tried to use it at skate parks and it's adequate. It's a good substitute for the Olympus E-5 that I was using for high school sports. Just like the E-5, the continuous AF is great. However, working at skate parks, nothing I have can beat 10 or 12 frames per second that the E-M1 and GH4 can do for capturing the whole sequence. Sure, the Samsung NX1 could do it at 15 fps but who knows whether it's going to be available or not.
I also bought a Panasonic GX8 and the behavior of the AF is similar to the GH4. Even with face detection enabled, it may detect a fence behind the person. That all changes when it's recording video, as with the GH4. I really appreciate having the GX8 and I've been using it since I sent the E-M1 for repair.