Guns: 9mm Carbine, Yea or Nay?

I've been thinking about building a 9mm AR, mostly because my broken elbow showed that it may in fact be easier to shoot a pistol-caliber carbine with an iffy right arm than to try to use a pistol solely with your off hand. Plus, there's the cool factor of being able to use an AR-pattern gun in an indoor range, or being able to shoot at close-range steel targets (relatively) safely:





Actually putting together a 9mm AR requires a series of befuddling choices. Unlike your run-of-the-mill 5.56 caliber mousegun, the magazines and operating systems of these 9mm critters seem to be in a state of flux. Like any firearm, I think you get what you pay for, so if you want to use a 9mm AR to potentially save your hide, I can't imagine you'd be satisfied with Bargain Bob's El Cheapo conversion that uses modified mags from some autoloader you've never heard of. The thought of a bottom-feeder with *shudder* $50-$100 Colt 32-round magazines strikes me as absurd, though.