As Greeny said. Steam might sometimes be more expensive than buying retail versions of games, but at least now I don't have to store a ton of game boxes everywhere. Or get off my lazy butt to buy a game
Also, it now features the "Steam Cloud" so it backs up user settings when you're connected online, so you can retain them if you use more than one machine (like me).
Edit: Regarding Macs, they've recently added Mac support... iirc there is an Apple section on Steam, showing which games are supported.