Be sure any mods are compatible with YOUR Glock. I love my 21sf, as much as I could ever love any inanimate object. If I had any idea the hassles involved in any upgrade with this one, I might have got a different model. Advantage Arms 22lr conversions, will only work if manufactured after March 2008, when I went looking last summer, nobody Canada or USA, had one that would work. Also, if you want a holster, you will most likely need to get one made. It took me quite a long time to find one to fit a Glock 21, only to get it home, and realize, it was not made for a Glock with a rail, such as mine. I've got one being made for me now from comp-tac.com
It is a very good, reliable gun. The only hassles I've gotten out of it, are some reloads with cheap plated bullets. I believe the problem, was poor bullet sizing, but seeing as Excel no longer is in business, that's not a particular concern (only a problem with 200gr SWC, the 230gr RN worked great).
Clean it, when it occurs to you. I rarely ever clean mine, maybe every 4-5 months, which usually works out to be 1000-1500 rounds, depending how often I could get out. Do not over lube it. Just the bare minimum in the places the manual tells you. I just use Hoppe's gun oil, nothing fancy is required, whatever you have will work.
Even using the cheap plated bullets, with inconsistant sizing, the accuracy is more than I am capable of. Lots of folk around here will tell you that your ##### will fall off, if you ever even think, of using cast lead bullets in a Glock. The 45acp Glock barrels, are 8 sided polygonal rifling, as opposed to 6 sided for everything else. 45acp is also a lower pressure round, with mainly subsonic velocities. These all contribute to, the careful use of lead bullets, with proper attention paid to any leading or pressure signs, being a viable thing to do. If you're not going to take the time to get to know your gun, don't do it, but once you know what you're doing, and what to look for, it's worth a shot.