Like any other pistols, including Glocks, the M&P's have had a few issues along the way. However, reliability has not been a huge issue. Like I recently wrote at LF, there were some problems with M&P strikers breaking during dry firing without the use of a snap cap, but the striker redesign has helped. Never seen any light primer strikes. Very early on (circa 2006) there were some issues with dead triggers, but those were quickly solved with a larger sear spring. Other than that, I've never had a problem with M&P triggers and in fact have always preferred them to Glock triggers--the lack of distinct reset is utterly a non-issue for me, as I cannot feel it when shooting quickly. There were some right side slide release lever tabs breaking off (note that this did not inhibit pistol function); the new part seems to have solved that.
That just leaves accuracy. All the M&P45, M&P40, and M&P9c I've been around have shot very well--on par accuracy wise with any similar pistols from other vendors. For me, the only M&P accuracy issues have been some inconsistency from M&P9's. As an example, we recently compared a dozen new OEM M&P9's with a dozen new OEM gen 3 G19's right out of the box after zeroing, using 10 rounds of Federal AE9FP 147 gr FMJ fired at an NRA B8 target at 25 yds slow fire. The best accuracy from the M&P9's was 98-3x; the best from the G19's was 99-6x. The worst accuracy from the M&P9's was a 90-0x and an 89-1x; the worst accuracy from the G19's was a 92-1x. In short both pistol types exhibit acceptable accuracy for duty use.
Now here is the big difference--the addition of a match barrel like a KKM to almost any G19 can significantly improve accuracy (like to 100-9x; still have not hit a 100-10x, but that is my goal this year), while the addition of a match barrel to M&P9's does not seem to improve things as much, if at all, in many pistols.