1) Yes, if you have time to play all the games you will accrue.
2)Prices are good with regular sales and then the two big yearly sales.
3)Chat with friends, Discussion groups, fan art, built in game guides written by fans.
4)Games, but they have started to branch out into offering programs and desktop software.
5)Obviously you need the internet to download the games, but you can play them offline once downloaded. It sometimes kicks up a fuss if it can't connect and won't let you sign in but there are ways around it.
6)It's free, but some games might want a subscription. (Basically some of the MMO's but most are going free-to-play now).
EDIT: Also, if you do decide to get Steam, here's a link for rules on surviving the big sales:
http://www.pcgamesn.com/how-survive-201 ... ummer-sale