I have personally owned an XCR with the 556 kit and 7.62 kit and had no issues with the rifle and was very happy happy with it. I sold it after the novelty of owning a blck rifle wore off, not really my thing.
One thing to note is that many people who say the XCR is junk are people who have never owned one. They saw it in a class or had a buddy own one or most likley are parroting other on the forums.
If you want to get must have extras, buy a bunch of LAR 10 round pistol mags.