I had a 12 gram CO2 quick changer on an external sniper rifle a while back and it was pretty nice. Mainly because of the lower profile, and you didn't have to go get anything filled. You could just buy a large box of CO2 capsules and be set for a while.
Unlike HPA though, CO2 will still have FPS variants. You will notice that CO2 is a lot better than green gas though. This is usually how it goes in performance: HPA>CO2>Green Gas/Propane.
In my experience, the CO2 cartridge will last around 50 shots or a tad more before the pressure really drops and makes it unusable. The pros of using the 12 gram CO2 set-up is that you can fit it in a pouch on the stock, or other spots on the rifle.