The advantages of CO2 are that it's compact to carry, convenient to use and gets up to full pressure FAST.

The disadvantages of CO2 are that you have to be very precise with your flat repairing technique and, when you run out of CO2 you're out.

When I was riding road bikes I relied 100% on CO2 for years and was never stranded. In the interest of full disclosure, I don't live in the goat head belt so I don't get very many flat tires. Since I switched over to recumbents I also switched to using a pump because my bigger volume tires need more CO2 than a 16 gram cartridge will provide.