I have an M&P 22 and I concur that they are worthwhile in terms of providing an understudy to the 9mm version. They fit the same holsters and generally feel similar.
If you just want something for static bullseye-type shooting, a conventional .22 pistol (e.g. Ruger, Browning, etc.) would be a better choice.
Some of my observations are as follows:
-It generally works well, but prefers fairly hot ammo.
-Frame construction is a zinc alloy (i.e. pot metal) chassis in a plastic grip frame. This is sort of chintzy and off-putting, but the overall design is still better than a SIG Mosquito or Walther P22, for example.
-Mine required fire-lapping of the bore in order to shoot accurately.
My ideal solution would be an Advantge Arms conversion kit, but they don't make one for the M&P, so the M&P 22 will have to do for now.