If you get the mini-CO2 cartridge system, be sure to get the "threaded" type. This will allow you to dispense just as little CO2 as you need for each use, and the cartridges will stay useable for quite awhile. The non-threaded type, you get one blast and it's all over. The threaded system will pay for itself quickly. I used these for years before finally going to a real CO2 system (which is completely worth it btw).
This is a good way to purge your carboys in advance, as bluesman noted. Much better to purge first, rather than just clear out the headspace afterwards. Just blast some in, let it settle for a couple minutes, and rack.