First hour of exploring: I'm really impressed with the ergonomics of the Olympus OM-D E-M1 (although Olympus marketing needs a lesson in product naming). If you are the type who loves to customize every button of your camera, you'll love the E-M1. The defaults are reasonable, though, so you don't need to customize if you aren't into that.
I love that I can set shutter and AEL behavior in every focus mode. I can shoot with focus on back button with full manual override w/focus assist (zoom and peaking), and use a single lever that changes to a second focus mode if the environment changes quickly. It also has AF Home registration, a feature I loved on my Canon SLRs. You can set an AF Home point so if you can customize your AF as much as you want and get back home with a single button press. Also, I can set an ISO range for auto-ISO (which can be set to work in every mode, not just P/A/S), and a max shutter time for flash (I find the 1/60 sec to be way too short, in most cameras—I set this to 1/20 sec).
It even has a large grip, now. I don't see any drawbacks vs. using an SLR, so far, except for lack of an optical finder for really dynamic or dark scenes (EVFs ruin night vision and are slow in low light). I have yet to evaluate AF performance, especially when compared to the excellent AF in the Canon 5D Mark III / 1D series, but the new embedded phase-detect elements in the sensor are supposed to make AF really good.
The EVF is excellent. So far, I'm finding it to be comparable to the Sony. It turns on instantly when you bring the camera up to your eye—no noticeable lag.