My department has had Berettas for years, first the 92FS, now the 96FS. The 96s are about 12 years old now, and most of them are pretty worn out. We were supposed to be switching to the Glock 22 (gag), but now it seems that S&W made us an offer on the M&P and it sounds like the powers that be are seriously considering it. Most of the folks want to switch to the .45 ACP anyway(me included), and if we go M&P, Smith is willing to trade out all our .40S&W ammo for .45ACP. Our huge stock of .40S&W ammo was the reason we were not going to switch to the .45 originally.
I carry a M&P45 at my part-time job and I couldn't be happier with it. I am hoping the deal works out for us.
I think that S&W did a couple of things to knock themselves out of the LE market over the last fifteen years or so. First, the Sigma was a flop, and its reputation has never recovered in LE circles since then, no matter how well the current version might work. Second, the all steel guns got to the point where they cost twice as much or more than everyone's favorite plastic gun, the Glock. Third, LE was moving away from DA/SA to striker-fired, and S&W didn't offer anything that fit the bill. I think the 4566TSW is about the perfect DA/SA duty gun, but for $1000+ retail, not a lot of departments were interested in going there.
Hopefully the M&P will get S&W back into the LE game big-time, it seems to be making progress so far. The two biggest city departments near me have made or are making the switch to the M&P45, as has one of the large Sheriff's Offices. The SO in the county I work in is currently in the process of switching to the M&P40, and State Probation and Parole went with the M&P40 in late 2007.