Like whit3 I also carried a Commander 1911 and shoot a full size in IDPA. wanted something lighter, faster, and NOT A GLOCK! Rented an XD, FNX, PPS, Taurus, Ruger and the M&P just felt so right. Best grouping of all of them and best trigger feel. XD(m) was close second but hated the feel of the boxy grip.I carried a Commander size 1911 clone for years. I finally got tired of battling rust (I sweat a lot) and wanted to go with something lighter than an all steel gun. I wanted a polymer frame pistol that had a rust resistant finsh. I tried several pistols, but the M&P fit the best in my hand.
My son has a SIG P320 9mm, and it's decent, but the M&P fits me a lot better and just feels better overall.I love the feel of sig sauer - but the price and availability was prohibitive - but then put my hands around the M&P full size and it fit like a glove.
Reasons for purchase - recreational shooting and home defense. I've even heard of ppl using the full size for conceal carry.