http://www.neogaf.com/forum/showthread.php?t=595606rbstewart7263 said:limit your shares to 4 times a month perhaps? id say that is fair.The Enquirer said:I don't think Valve will pull some crap like mirosoft with DRM's. I think they will actually say "ok, if you want to share some games go ahead". I think there will without a doubt be limitations on this because it really doesn't make sense to let tons of people play a game from one purchase. As cool as that is it would bankrupt the company and we really don't want Ea becoming the primary digital distributor. Realistically I just trust Valve on this when it comes to doing something cool and the right thing. I doubt they will be backing out unless they can't get it to work properly, which in itself is very unlikely.
According to what I read from this, it looks like 1 person at a time will be able to borrow the game. Same thing with a physical copy. Unless you have more than one disc only one person at a time can use it. I'd bet that this update or maybe the next one they will implement something where you can play co-op with the person or something like that.
Granted this game sharing could be very touch and go for a bit depending on which publishers don't like it. I suppose though this is a huge advantage of technically not owning the game.