Some of the low serial numbers had issues that S&W seems to have resolved with updated springs and a slightly improved magazine. Like all firearms that are relatively new to the market, there are minor issues. Overall, the 15-22 has proven to be an exceptionally reliable rimfire rifle which functionally comes closer to the true AR platform than any other on the market.
The magazines are demanding only in that you load them properly to ensure consistent staggering in the box. With proper loading, mag related problems are few and far between.
Ammo is ammo... some manufacturer provide better quality, some sell you junk. The preferred ammo among this community includes Winchester, Federal bulk (Walmart 550 packs), and anything CCI (Blazers, MiniMags). Generally avoid stuff like Remington RGB (Remington Golden Bobbleheads) and a few others. The 15-22 owners manual actually lists recommended ammo as well as stuff that has proven to be unreliable.
I personally have made the decision to stick with CCI ammo. The Blazer ammo (not copper washed) tends to get your hands dirty while loading mags, but fires reliably and consistently every time.. no perceivable variation in the *bang*. Blazers can be found on sale frequently at prices that come close to the Federal bulk packs. CCI MiniMags cost a bit more, but being copper washed are cleaner to handle... and just like their less expensive brothers, fire the same way every time. There are a variety of rounds that the 15-22 will gobble up reliably.. what works best for you will probably take a little experimenting.
I've had my 15-22 for a little more than a year with no problems, no defects and after MANY thousands of rounds it just seems to be getting better!
I wouldn't hesitate to recommend the 15-22 as a solid purchase... hope this helps and whether you buy one or not, welcome to the 15-22 sub-forum