CO2 tanks are great, I've had one since 2006. But, they are very heavy, and really should be secured solidly in the truck so that they don't go flying around. I actually stopped taking mine on the trails years ago, just ran out of room to keep it secured with all the other stuff I have in there.
Keep in mind that the pressure gauge (if you even have one, depending on the regulator you choose) won't tell you how much CO2 is left in the tank. An easy way to tell when you're getting low is to weigh the tank. Weigh it empty before you get it filled the first time, and then weigh it again right after. A 10-pound tank should be 10 pounds heavier, more or less. When it gets close to the empty weight you know it's time for a fill. Also, at least in San Diego, the places I've been to won't top the tank off, they empty it, test it, then fill. That means no going in with half a tank for a quick top-off.