Is the M&P15-22 series also using direct impingement, just like the 5.56mm version?

Never mind, it works via blowback method.

For those who have had it for some time, does the polymer upper/lower last?

Yep or at least mine does. No problems mounting lights or grips, because of low recoil scopes fare well also without eating away at the polymer. I've dropped, slammed and scratched mine in various mishaps and can't tell it from NIB except I've not cleaned the bolt or chamber for about a 1000 rounds and that part is getting grungy but it still runs like new. YMMV

I haven't had any issues with my 15-22. The polymer upper, lower, rails are sturdy.

The only caveat is to be careful with cleaning solvents. Stay away from Breakfree Powder Blast, Birchwood Casey Gun Scrubber, brake cleaner, etc.

Birchwood Casey Synthetic Safe Gun Scrubber works well. I've switched to Slip 2000 non-toxic cleaner/degreaser.