I have a DPMS (2009 production) that is very nice and does what it's supposed to. Only issue, is doesn't like PMAGS. You can get one in the well if you really really try but it wont come back out. From what I can find out, they changed a few things last year. Probably trying to capitalize on thier high mark up mags.
I have one I built from a CMMG lower that loves the PMAGS. Older DPMS also seem to love teh PMAGS. And no, it's not just my paticular rifle. Seems to becoming more common. I know of a few others as well. DPMS claims its just tight tolerences.