Love it. M&P centerfire shooter here, but more than a few of my shooting pals who are not M&P centerfire owners really like them, too.
Mags will become available again, you can backorder them from Brownells. If you look around, you will find some Canadian retailers selling the pistol with two mags, or with an extra mag available with the pistol purchase.
Elevation adjustment with all 22 firearms has to factor in the type of ammunition since there is such a great variety available. Straight-blowback semiauto pistols (like all of those described here) are probably most prone to POI shift due to ammo.
For longevity and straight-up accuracy on a budget, you probably won't beat the Ruger. If you're a tinkerer, you might enjoy a GSG 1911-22 most, since it uses many of the same parts as centerfire 1911s. You can make those triggers to feel amazing! But honestly the M&P22 trigger is no slouch, either. It is an internal hammer single-action-only fire control system. The trigger on my M&P22 is MUCH better than that of my M&P9, which honestly is the biggest limitation for using the M&P22 as a trainer for the centerfire pistols.