I've been pretty impressed with the DPMS one I bought a few weeks back to practice with for high-power. I'm not tearing up the X-ring of the MR-31 target by any stretch of the imagination, but when I screw up, the hole's in the direction that I screwed up in, so I'm the limiting factor and it's accurate enough for me to learn. I'm sure, however, the CLE and Accuracy Speaks are really, really nice. Some of the ones mentioned so far (the DPMS, CLE, and Accuracy, possibly others) are designed to mimic a Service Rifle upper, and are quite heavy with float tubes under the hand guards to keep the sling pressure from moving the point of impact.
There's also the Mk261 (I think) insert that uses your upper but changes out the bolt and carrier and magazine, but the rest is the same. Those are substantially cheaper, too. (But I'm happy with my DPMS .22 upp er)
Nate
Edited to add: I haven't used my DPMS with any normal-velocity .22, but it works darn with the blue Federal High velocity ($1.07/50 at walmart), and with Federal's "Auto-Match" that's $11/325 at walmart. Oh, and right now it's on a Bushmaster lower as well.