I always have these with me instead of a pump, much quicker and easier - especially on cold dark winter nights.
I use Genuine Innovations ones and have never had any problems with them - just make sure you keep your gloves on cos if they freeze to your skin it'll hurt !
Also, if you buy the replacement cartridges in bulk off ebay they're a lot cheaper.
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I have one. They're hugely convenient but make you lazy. Then, they eat money. If you only carry one or two carts on the bike and you fubar fixing a puncture a couple of times then you're out of CO2. Still, I like having mine for a quick top up out on ye trail.
I carry a couple - only used once so far on my roadie. Got a box of 12 cylinders for about £6 off ebay.
Have a simple brass adapter approx 1" long and threaded at each end, which sits between the cartridge and the presta valve. Simple, cheap, light, effective.
Just haven't yet found similar simple adapter for schrader valve
Oh - I also carry a minipump and spare tube and puncture repair kit