Another vote for the Morph, if you want a pump that can get your tires to 100psi.

CO2 is great for fast rides and races and can also be a real life-saver when late for work or for a woman riding alone who just needs to get rolling quickly.

The downsides?

They can be expensive ( although the non-threaded canisters can be bought more cheaply in bulk at your local big-box discount store).

They are wasteful (anyone know of anyplace that recycles those suckers?).

And the big downside: you still *should* carry a pump. Yes I've been on rides where the two or three cartriges were not enough. There are a couple of CO2 devices that also function (barely!) as a mini-pump in a pinch.