Tried one yesterday when a mate popped round with his (using the 35mm lens).
Feels VERY similar in use to the Df - that means that all of the criticisms aimed at the Df in terms of acting like an old SLR with the dials etc are just as valid here IMO. That's neither good nor bad, just a statement.
He also had an XE-2 here and we couldn't really see much difference in AF speed in low-light. It tracks better using the AF points that use the PDAF, no doubt about that, but as soon as you're outside into the edge AF points it's no quicker that we could see in real terms. The Df did kick it's arse there (which means a D600/D610 would) and the D3s absolutely slaughtered it (both using the Nikkor 35/1.4). Moving the AF points around isn't as quick as just hitting a d-pad but there's no reason why that couldn't be sorted with firmware. IQ-wise we could see no difference against his XE-2. The DSLR comparison was done because that's what I'd be giving up and sadly it's not quite there yet for me in terms of a camera I could use for 40+ weddings a year, some in the very low light we get for UK winter weddings. I'm also not quite ready to give up a good OVF.
BUT - very light, very compact and very well built (on a par with the Df I'd say, but obviously smaller and lighter). It's certainly an excellent camera and extremely well-priced.