They're best used as a get-me-home (or get-me-to-the-Finish-line) measure. For one thing, CO2 quickly seeps through butyl rubber inner tubes, as strange as that must sound... fill a road-bike tire to 80psi with a CO2, and it'll be really soft the next day. Also, if you have more flat tires than you came equipped for, you'll be walking. I usually take an inflator and a small pump.