I have been running an XCR as my primary carbine since they came out a couple of years ago. I have one of the early ones. I had it cut to 12" (SBR) from the get-go, so I couldn't tell you what accuracy was like out of the 16" and 18" barrels.
The only differences between the newer ones are an improved firing pin (mine snapped at 3500 rds, replaced for free), a toolless gas valve, and an integral trigger guard.
The XCR is a VERY reliable carbine provided you do a few recommended procures like break-in, and loctiting of the ejector screws. The match 2-stage triggers just came out.
Pros
-Reliability
-Integrated monolithic rails (aluminum extrusion, one peice upper receiver)
-Hot-swappable quick-change barrel
-works with most AR accessories
-Designed from the ground up as a piston rifle - was not retrofitted
-Bolt hold-open is a very intelligent design - ambidextrous, can be actuated with the tip of the finger without changing sight picture or cheek weld.
Cons
-Price * although, this is not that big of a deal considering what you get versus a top-tier AR and rail
-Parts availability (only really can be had through Robinson Armament, but their CS is top-notch, so no big worries here, either)
-Center rail lightning cut (this is a personal one for me - it is technically out-of-spec, and the lightning cut chews up center-mounted pressure grips, like the KAC and ProMag)
-Weight: with the Heavy bbl (even with my 12" SBR) these are a bit heavier than you would expect. Add a bunch of accesories and you will go to muscle failure trying to run one for extended periods of time
-FCG: these are not the same as an AR. It is a basic AR design with a slight offset to the right. AR parts will not work here.
RobArm is now offering a 1/7 barrel. It used to be only 1/9, and the early ones like mine had 1/8 bbls. The Micro PDW and Mini PDW are also now offered, which basically include a shorter piston system and upper receiver/barrel. These are as slick as owl shit.
ETA: PM XCRMonger on this board. Terra works for RobArm and handles the CS....she's good people and has helped me out several times. If she can't sell you on buying one, I doubt anyone can.