I use a S&W M&P 15-22 as my trainer/practice rifle for my AR's. It's the gun I use whenever I'm teaching someone on the AR platform or if we're practicing since it keeps costs down. All of the controls are exactly like the AR-15, even the procedure for breaking it down for cleaning is almost exactly the same. The only thing it's missing is a functional forward assist, which admittedly I've never actually had to use on any of my AR's. It would be nice if it had one but it's hardly a deal breaker, plenty of military AR's don't have the forward assist either.
We've easy put between 5-10k rounds of 22lr through our M&P 15-22 and it still runs like a champ, that's even with dumping the mags on the ground and getting them fouled up with grass and dirt. Pretty much the only time it hangs up is when you get a dud round from one of those bulk packs. I think we may have managed to get it dirty enough to start fouling up once, but it took a solid 3-day weekend of shooting without cleaning to do it. A thorough cleaning afterwards had it running at 100% again.
I wouldn't hesitate to suggest the S&W M&P 15-22 to anyone looking for a solid practice or training platform for their AR-15. In that role there really isn't anything else that compares.