Steam's customer support ranges from good to nonexistent, depending on the issue. They do not offer tech support for any games; you have to contact the original company. They - in theory - have a blanket no-returns policy, but people have received refunds on a case-by-case basis by contacting Steam support. Steam has an excellent friends/community system which is integrated inside many games offered on Steam. Most games also support the Steam overlay, which offers access to the Steam community service
As long as Steam operates, you can re-download at any time. You can backup your files as many times as you want. You can install the game on as many computers as you want.
However, Steam games requires that you be running Steam and that you are logged into your account when you launch the game. Only one computer at a time may be logged into Steam.
Steam - very rarely - has outages for server updates, which has been known to kick people out of games (though I believe that has been fixed.)
Steam is also known to have odd pricing policies. Some games can be gotten very cheaply, particularly during their periodic sales. But other games cost significantly more than they do in stores, particularly on the European version of Steam.
