Interesting that you would mention that. The initial run of G21's had issues, and mine was one of the most unreliable pistols I've ever owned. Mine seldom made it through a box of ammo without choking once or twice.
Glock seemed reluctant to admit there were problems with the new gun, but they eventually started requesting that just the slide be returned. When asked, they wouldn't reveal what mods were being done to the slide, so I sold mine and didn't buy another one till all the bugs got worked out.
My second one (bought a year or two later) has been very reliable (it has only malfunctioned once IIRC) but it's not particularly accurate. In an attempt to address this, I purchased a Wilson Combat Match Grade barrel. Not only did that not improve things, but it also proved to be less accurate than the OEM barrel with some factory loads. Anyhow, the gun proved more than accurate enough for me to kill a couple of deer with it, so I haven't lost any sleep over the matter.