I bought a Lone Wolf 9mm conversion barrel for my G22, along with a G17 RSA. I use the stock extractor and .40 magazines. It has not run perfectly, but is more than adequately reliable for range work.
I now have a KKM conversion on the way for my G23, along with a G19 RSA, and also a 9mm mag for each gun (along with orange baseplates to avoid magazine confusion). I guess I'll find out if the cartridge appropriate magazines help reliability or not.
The reason for the conversion barrels is two fold. Firstly, 9mm is cheap and low recoil, so learning to shoot quicker with accuracy is easier. Secondly, it's not a bad idea to have some caliber versatility, but I don't necessarily want to buy other guns just for that reason.
I usually carry and practice with .357 Sig in these two piatols using KKM conversion barrels. They are very good. The stock .40 barrels don't see much use these days.