Cool! They sure are popular. In the couple of years since I've been shooting a "tactical .22" rifle match, the MP-22s have become the far and away favorite, displacing the 10/22s. I think the fact that they could be had for around $400 (before the nuttiness) and didn't need/warrant the customizing the 10/22s got enhanced their popularity. About the only thing the base gun needs to be competitive in our club matches is an optic, and Primary Arms has a $60 clone of the Millet DMS which seems to cover the bases nicely.
About the only thing that fouls these guns up is running drum mags (50+ rounds). We have one precision stage with targets from 15 to 100 yards that typically takes about 60 rounds if you have no misses.. The "hosemaster" crowd goes for speed over accuracy so of course they run drums. I don't think I've ever seen a drum mag run from full to empty without a malfunction of some sort, but the single stacks seem to work just fine overall.
We had a guy a couple of matches ago running a suppressed .22 AR - not sure what it was. He had to run 3 of the 4 stages over because the timer wasn't registering his shots - that's how quiet the gune was!