It takes about 2 or 3 of the little 12 or 16 gram CO2 cartridges to push out a full 5-liter keg of homebrew. At less than a buck a pop, it's not a huge expense, but worth a small amount of consideration.
in my experience it takes about 3 16 gram CO2 cartridges to push out 5 gallons of homebrew but i have only experienced this when friends are around and the keg gets finished that night. If your pushing your 5L keg over a few days there could be few areas of CO2 loss
the CO2 devices were not really meant to hold pressure once the seal of the cartridges has been broken. The 16 gram cartridges were designed to hold enough CO2 to get the full 120 psi into a road bicycle tube in the event of a flat. If you only use a portion of the CO2 cartridge (as homebrewers do), , there could be loss of pressure within the unit after a day or two. Also, from my bike shop days, the french sytle or presta inner tube valves can lose pressure unless you screw down the top.