I picked up a XCR a few weeks ago and I'm quite happy with it. It is very similarly configured to my RRA flatop AR and the weight feels about the same.
XCR
HBAR 16" barrel
built in midlength quad rail
Tango Down pistol and vertical grips
Aimpoint Comp M4
Surefire 660
AR
HBAR 16" barrel
Daniel Defense 9" (midlength) rail system
Tango Down pistol and vertical grips
Aimpoint Comp M3
Surefire 660
One comment from my rifle league friends was that it's very loud compared to others running their ARs, however, during the first 300 round break in period, Robinson recommends running with the gas system wide open (position 4) and using hot ammo like American Eagle. I've been running Q3131A. That might be contributing to the loudness.
Otherwise the rifle has a comfortable natural firing position, satisfying and solid feel to the action upon firing and very exuberant extraction.
As far as the POF systems, all things considered, I'd rather go for a system that was designed from the start as a gas piston system than bolt one on to a DI. I've no doubt that some wizbang engineers can make it happen, but I'm following my own prejudices on this one.
A side note: I'm a BIG fan of Sig. The company, their handguns, their bolt guns and the original 550 series. I own 5 of their handguns, a SIG SHR 970 in .270 Win and a Sauer SSG 3000. I've shot the 551 and 552 a number of times. I've been looking forward to a domestic 550 style rifle for 10+ years now. But comparing the 556 and the XCR, it just seemed to me that the latter was better thought out and better executed. The ergonomics are better, the gas system more sophisticated and suitable for suppressor use, and I'll even be able to swap calibers on the XCR and try out the 6.8 SPC or 6.5 Grendel for under $600 (haven't decided which yet
) I'll still get the 556 eventually, but I'm hoping that they make some modifications to bring it more in line with the 551 I know and love.