The only "new" S&W's I ever had troubles with, were two 940's (9mm Centennials, one of which was the replacement for the other). Both guns failed within the first 200 rounds, and both had to be disassembled to get the live ammo out to send them back.
Smith did take care of it, and in the end, sent me a 642 as a replacement, since I didnt want another 940 at that point.
The only other problem, was an old M&P .38, and it was well worn when I got it.
Other than them, all the others (around 20-25 guns) have been fine.