Let me say this: I have ridden in club rides where some riders had the gas-style units. If any of us had a flat, inevitably we waited for one of the gas people to get out his or her kit and do the inflating. It was just so much faster and easier than banging away with a pump. A typical inflator is the Innovations Ultraflate, available from Bike Nashbar for $24 (www.nashbar.com), including three gas cartridges.
That said, I don't use one for everyday riding. I've seen too many instances where I (or someone else) think the flat is fixed, but in fact there's still a hole in the inner tube. So that wastes a cartridge. And while the cartridges aren't a huge expensethey're typically around $2 a popthey are, nonetheless, an expense.
Besides, air is free. I just think pumps are cheaper and more reliable in the long run, particularly on those rides from hell when you suffer three or four flats over the course of 30 or 40 miles. As for which pump, for road bikes I remain loyal to the super-reliable Zefal HPX Frame-Fit Pump ($25; www.zefal.com). Very hard to beat. Topeak's Road Master Blaster ($30; www.topeak.com) is another good one. For mountain bikes, Blackburn's AirStik ($25; www.blackburndesign.com) is pretty good, too.