Upload & Sell: On
I went from a 5D2 with a bunch of L glass, back to a Nikon D7000 (trying to save size/weight) and DX glass, and was unhappy with the quality of the DX glass so ended up with FF compatible glass, and then a D600, then a D800...at which point, I had a kit that was even heavier and more complicated than the 5D2 I sold because of that exact reason.
Sold (or in the process of selling!) it all for an OM-D and some of the choice, nice m4/3rds glass (7-14, 25/1.4, 45/1.8, 60 macro, with the 75/1.8 and 12/2 to be added also). This entire kit is basically the same size/weight as the D800 with one lens. It's stunning the difference in size.
Size/weight is one thing, but most importantly, the IQ of the OM-D is at *least* equal to any APS-C DSLR that I've shot (most of the Canon's, and the D7000), if not better. The image stabilization is so effective that it exceeds (yes, I said "exceeds") my FF Nikon for available light shooting of static subjects, and at ISO's up to 3200, noise is very, very well controlled and can be cleaned up a bit more in post also.
Yeah, the buttons are fiddly. It's not that big a deal, really, but if I was shooting events professionally, I'd want a proper DSLR still, though I'm excited to see what Oly does with the "pro" EM-5 that's rumoured, as I suspect that will end up in my bag as it would, for me, truly hit the sweet spot. The AF on my OM-D is brilliant for single-shot. I've read that it sucks for continuous tracking, which I hardly ever used on my SLRs, so it's a non-issue for me. I can say confidently that the single-shot AF (*especially* in low light, where it is a lot better than the Nikons) is quicker and more accurate than my D7000/D600/D800, and is in a totally different league than the 5D2.
Like a previous poster, I have (my wife's) RX100 as a truly pocketable walk-around, and it's phenomenally good for a P&S. Rich, detailed RAW files that withstand a surprising amount of pushing in post.
I'm extremely happy with this setup now (RX100 in the pocket for walkabouts, hiking with the kids, and casual shooting), and the Oly kit with a handful of high quality primes for my "real" camera. I would like to say I'd be interested in a FF mirrorless but the huge advantage, IMHO, of m4/3rds is really the LENS size, not so much the body. I'm guessing a FF sensor would require glass compatible of projecting an image circle onto that big sensor, and then the size/weight advantage would quickly be lost, no matter how small the body is.
Definitely check out the RX100 if you want a pants-pocket walkaround. If you're truly looking for an interchangeable lens camera as an alternative to a DSLR, I'm tickled pink with the Oly system so far.