Many people report significant loss of inflation in one day. The CO2 molicule is soluble in butyl rubber so it sneaks through the interface by a method other than leaking. It works good for on the road/trail fixes but should probably be replace by real air upon return to home.

The 16 gram cartridge will fill a 700c 23mm tire to about 110/120lbs and a 26 x 2.0 to about 40psi.

My own preference is for the Planet Bike Red Zeppelin or any other inflator that screws on to the tube stem, uses threaded 16 gram cartridges (more expensive) and has an on off adjust knob to regulate flow. Those that are pressed on by the user are prone to leaking and giving you a hard time. 12 gram cartridges are just large enough to require 2.

Beyond that, people who suffer multiple flats on a ride can run out of cartridges.

http://ecom1.planetbike.com/1019.html. Look here for device and manual including psi/size chart.