As an owner and armorer I've posted a fair number of answers to this sort of question about the M&P pistol line. I don't have the time this morning to try and recreate those postings in this thread, so suffice to say I think S&W has a real winner on its hand with the M&P pistol line.
Having attended something like 7 armorer classes for 3 other plastic-framed pistol product lines, I've come to be of the opinion that S&W has adopted some interesting and useful refinements and features in the M&P.
Yes, they've been busy making some revisions and some assorted improvements (and identifying some vendor issues) since the model line was released in Jan '06. They aren't the only major firearm manufacturer who have had to make ... or are still having to make ... some revisions and changes in their plastic-framed model line.
Some folks have complained about the trigger reset. Personally, I don't train myself or anyone to shoot to "trigger reset" when it comes to defensive drills and skills. Then again, I'm not a sport/target/competitive shooter.
I've seen more shooters than I can remember experience unintended subsequent shots on a training/qual range when trying to release a trigger to its "reset point" and hold it there before they've actually decided whether they should fire another shot. Sometimes it's seemed like it was due to being distracted and under a little stress. Sometimes it seemed as though it was caused by hand confusion, or being startled by another shooter on the range nearby. Sometimes it seemed like they "rocked off" an unintended shot due to trying to manage recoil while trying to release their trigger just to the "reset" point.
Whatever the causes may have been, you can bet that if those unintended rounds had been fired off the range, in the real world, and had injured or killed someone that should not have been shot, it would have been an issue. I wouldn't want to be the person to try and explain on the stand that I'd been violating one of the most stressed rules in firearms handling because I wanted to keep my finger on the trigger to keep it at the "reset point".
That's just me, though ...
The triggers seem to improve with usage. The standard factory tolerance for trigger pull is 6 1/2lbs +/- 2lbs for the 9/.40/.357 models and 7lbs +/- 2 lbs for the .45 guns. My M&P 45 was on the distinct heavy end of the acceptable range, and a bit rough when new, although that didn't stop it from exhibiting a predictable break and excellent accuracy. By the time I checked it again at approx 2,500 rounds it was feeling much smoother and the gauge showed it to be producing averaged sets of trigger pulls from 5 1/2 - 6lbs (stock).
I have other thoughts, but I have to go.
I think it's a great design which is quickly becoming better and benefiting from increasing refinement.
I'm planning to buy a 3rd one next year.
I enjoy shooting my M&P 45 more than my various 1911's, FWIW. Weird, but there you are.
I'm also considering ordering a Performance Center 1911 and/or a PC 945, too, so I'll add to my metal-framed .45's if all goes well.
I own 9 .45 pistols, and the M&P 45 is the only plastic-framed one in the bunch ... for now.