I have had CO2 inflated tubes completely flat the next day, at least twice. Other times, it was only partly down.

A couple more pros/cons, skipping previously mentioned items:

Plus
Size - obvious
Instant inflation.
Pump failure less likely (I have been stranded 3 times with broken pumps)
It is harder to break off a valve stem with CO2 (but I have done it by pump & CO2)

Minus
It is harder to partially inflate the tube for installation. You can do this by mouth, but pressure is limited.
If the metal CO2 cartridge is exposed, it gets dangerously cold.