There's another thread running on this same topic; maybe moderators will merge them. For now, I'm quoting myself from that thread:
"I just saw it in person yesterday at "The Gun Store Las Vegas" (which incidentally is a great place to visit). The person helping me was quite friendly and even let me dry fire it. The trigger pull is maybe a little shorter and crisper than a regular XD, but not by much. (I own an XD9) The feel of revised checking/texturing of the grip was, similarly, slightly improved but no big difference, and the slide serrations were such a minor change that I didn't notice any real difference in feel. IMHO the major benefit of "M" is that the .40 mags hold 16 rounds, which is a lot for a .40, especially since the gun seems nearly the same size as the original XD40. Probably a great gun, but evolutionary rather than revolutionary. I agree that it is probably aimed mostly at the law enforcement market."