I just started using steam this weekend - typical I chose the weekend where loads of people were having problems with the shopping cart -thankfully there's a workaround....
Steam have said they'll make the games playable if they ever go under, but by my reckoning if and when that ever happens, you'll be unlikely to still be playing your back catalogue anyways....
It also lets you pre-download games so you can activate and play them the minute they're released which is nice, and you can play them on multiple computers, and reinstall easily if you ever reformat your hdd, which is nice.....