Say Uncle and Tam both mentioned their newly found appreciation of the concept. I've mentioned I've been a proponent off and on before as well, and "Smith & Wesson M&P15-22" (or some variation) is the second most common search subject that hits my blog, right behind the Sig E2 series (why that is the most common I'm still not sure...)
Both of them are using a conversion bolt in a standard AR, which is probably the quickest and cheapest route to go. I haven't had any personal experience with that particular setup, but Tam does bring up one of the various reasons I didn't go that route myself, in that the .22lr's habit of leading up your barrel is a bit of a problem with a barrel with a gas port inside of it. She does try to use copper washed bullets as much as she can to fight that, which is really the only thing you can do, other than bringing along a bore snake to run through every few mags (something I tend to do with .22lrs anyway.) This also caused her to buy a box of Remington Gold Bullets, which in my experience, is the least reliable bulk .22lr ever made, and is still dirty as hell, despite the copper washed rounds.
Another option out there is a dedicated .22lr upper, such as the Spikes Tactical units. I was originally going to go that route until I found out about S&W's military discount program. They do have some sweet rigs available, and you can deck them out with all sorts of goodies. This month's SWAT magazine has a sweet suppressed SBR one with a 5" or so barrel and a YHM can almost completely covered by the handguard. It is a seriously sweet looking (but quite expensive) gun. They also generally use Lothar Walther barrels, so even that stubby little one is pretty dang accurate.
But in the end, between the price of the Smith, even without the discount, and the ability to use the bolt release, I'm pretty happy with the little plastic popper. Still need to get a decent sight setup on it though.
So that's a good sign. Robb Allen mentioned that he ate one, so he's due to keel over at any moment too. I had one after doing some cartography work last night, because I was hungry, I like bacon, and I'm trying to cut down on carbs (heh). Overall it didn't strike me as anything too exciting, really more like some lazy person just slapped some stuff together, figured it's different enough to sell, and threw it up on the menu. I wouldn't say you should avoid it due to the potential "health hazards," but the price is, in my mind, not worth it. It's like $5 and is quite bland for a fast food product at that price level. Maybe I'm just spoiled by all the $1 value menus...