I'm fortunate to say that I haven't had to purchase a whole lot of these. However, all it takes is one ride to remind you that you might ought to have a few extras on hand. I use the Portland Design Works Shiny Object inflator. These thread in securely and stay put. They also fit in my small under-saddle bag, which holds a spare tube, levers, inflator, tire boot, etc. I find that these will only inflate a 25c tire/tube to about 80-85 psi, which is adequate to get through the ride. If you happen to have a hand pump and get the tube partially inflated, then these are great at topping off to higher pressure. It all depends on what your tire/tube situation is and what you're willing to carry on a ride.
Tip: Cut a portion of an old tube and put it around the C02 cartridge. These things frost up immediately as they expend their compressed CO2. So, after mounting your tight tire back on the rim of your expensive carbon wheel, it can hurt when your tenderized flesh pulls loose from the ice-cold surface of the cartridge. An old tube will protect your skin and makes the cartridge just a bit less bulky in your saddle bag or jersey pocket.
I always carry a pair of CO2 inflators in my under seat bag. Normally I can only find 12 gram size. 20 gram cartridges are too tall to fit in & close my under seat bag. The larger 16 gram just make it. I carry 2 in each of my road & cycloross bicycles. So the six pack was perfect for re-outfitting for the season.
On our broken glass covered streets, these are a life saver. quickly getting me back on the road without the added energy expenditure of trying to inflate a tire with a mini pump after repairing a flat. Still a good tradeoff to sloppy tubeless obsession, which I've tried and given up on.
These are reliable CO2 cartridges that last until you need them.
I've gotten cheaper ones before that turn out not to work when you
need them. These have always worked when I need them.
Cartridge inflation is useful when you need save space, weight, and time. I keep a fresh tube and inflator system ready to go on my road bike that has scarce room for a pump. Otherwise, this is a commodity product that does what it's supposed to do.