What are advantages and disadvantages of CO2 inflators over frame pumps? I have a frame pump that I've used on the road when replacing flats, but I always see CO2 inflators advertised at bike shops.  Are they worthwhile for the potential weight savings?
Advantages:


Fast - generally these fill up your tire much faster than you could with a pump
Less effort - hook up, push button, no pumping
Possibly less weight (but I haven't compared weight of smallest pumps to CO2)
Lower initial investment - a valve and 2 cartridges generally seems to be cheaper than a good small pump.


Disadvantages:


Consumable cartridges - which is a cost issue if you have to fill up often
No backup - if you patch a flat and don't get it quite right, you don't have a way to fill up anymore.  You could carry extra CO2 cartridges, but then you could lose the weight advantage.
Limited on what tires they work with. Big tires will get less pressure.
Less friendly - if your friend gets a flat and you've got one cartridge, will you loan it? If you've got a pump it's no problem at all.  I've helped out complete strangers with my pump before.
Goes low faster. CO2 seeps through the tube a bit faster than air, so you'll have to top off the tire sooner than if you'd used a pump.


One possibility to consider is a combo unit.  Generally the pump on those is terrible, but it should be enough to get you home if you run out of CO2.  Or carry both a pump and a CO2 valve and a cartridge.  That way your first flat can be fixed super-fast and you avoid all the disadvantages (and the weight of a simple valve and one cartridge isn't much).