I relied totally on CO2 for years and it never let me down.
To me the most important thing is not to rush. CO2 is really fast so it's tempting to show off by fixing your flat faster than any onlookers have seen before. I think it's better to take time to be sure your tire bead is seated and your inner tube is out of the way before hitting the CO2.
I'd still be using CO2 but I started riding recumbents with higher volume tires. I'm too cheap to spend 2 cartridges every time that I get a flat.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.