I traded a CZ-452 Style for a NIB M&P15-22. It's had 1500 rounds put through it without a problem. Haven't had any optics on it yet, but with CCI Mini-Mags and Federal Auto Match, I was shooting 3-inch groups at 50 yards offhand without a sling. I ordered a cantilever mount for a 8.5-25x50 scope to see how good it really shoots.
The butt stock was a little loose, so I used some aluminum tape on the faux buffer tube and it tightened the stock right up. No wiggle at all, now.
As far as being like a "standard" AR, it certainly isn't, and here are some of the reasons why:
0) The upper won't go on a standard AR lower
1) The buffer tube is not a real buffer tube
2) The hole for the rear takedown spring/detent is at the bottom of the lower behind the trigger guard. This is probably because of the buffer tube being fake and the gun not needing an endplate, so there's nothing to retain the takedown pin spring.
3) The barrel/barrel nut is not the same as an AR-15.
4) The barrel cannot accept a gas-block type of front sight because it's not big enough. It's .685, and the AR profile is .750 inches.
5) You have to buy the right version of the rifle to get a threaded barrel that accepts a flash suppressor. IMHO, this is complete unneccessary (a .22 doesn't need a flash suppressor) but hey, if it's looks you're after, you have to weight that decision.
6) The handguard is not a standard AR-15 handguard, and your only option if you want one is to change the barrel nut, and then you can buy the desired AR-15 floating handguard.
7) Pistol grips with a beavertail (like the Magpul MOE grip) don't exactly match up well to the back of the lower, and you'll have a slight gap there as a result.
The bolt will lock back on when firing the last round from a magazine, and the bolt release actually performs like an AR-15 bolt release. I don't think the Colt has the same functionality, and I'm not sure about the Sig at all.
It was designed and intended to be light, and not having a heavy bolt, a recoil buffer and a lighter barrel go along way to that end. The rifle certainly costs less than buying a dedicated upper for an AR-15 lower, and when you stop and consider that the rifle certainly isn't a match-grade rifle (nor is it intended to be), it's a fine representation of the AR platform, and it seems that S&W has finally gotten a grip on their reliability issues.
As far as what ammo it "likes", I can tell you from experieince that EVERY .22 rifle - even rifles of the same model/manufacturer - has a different preferred diet. I've shot CCI MiniMags and Federal Auto Match though mine without any probolems, but I have some bulk 550, Wolf Target, a couple of types of Ely, and others that I can try.
I believe that the rate of twist on the 15-22 is 1:14, so this means that lighter bullets (30-40 grain) will likely work the best in the rifle. Anything much heavier will not stabilize as well. Another factor to consider is the speed of the bullet. S&W recommends not using subsonic bullets because they don't produce enough gas to work the action correctly.
My advice is high-velocity (1200-1300 FPS) stuff like CCI MiniMags at around 40 grains, or hyper-velocity (1400 FPS) like CCI Stingers, or even better, the CCI Velocitor. I'm not neccesarily a CCI advocate, but next to Federal, they are pretty much the most recognizable and easy-to-find brand out there, and the bullet velocities are well published, and I get really good results from them, regardless of what rifle I'm shooting. (I also have a Henry H001T and a CZ-452 Silhouette.)
My overall impression of the 15-22 is that for what it is (and what it isn't), it's certainly an adequate .22 rifle and if current interest is any inidcation, we can probably expect to see custom barrels being made for them (18 to 22-inch, and with faster twist rates) sometime in the not-too-distant future.
I personally like the rifle.