I can't speak directly to the cleanliness of these kits as I don't personally own one. If you use good ammo, however, I don't see how these would be any more dirty than a standard .22 LR pistol.
It appears there are several kits available including those made by Advantage Arms, Tactical Solutions Gear & Ciener. The brief reviews I dug up recommend using CCI MiniMags. Several guys over at another forum recommend the AA kit. Someone actually has an entire webpage devoted to bashing the Ciener kit (actually the company as a whole).
Advantage Arms has a recommended ammo list:
Advantage Arms Secure Online Store: Glock Recommended Ammo
I've been using the Remington Golden Bullets and have had no issues... most 22LR ammo is dirty, but I have had no issues with malfunctions. The copper washed bullets don't foul the bore as bad as regular lead.