I know that everything that smells of copy protection isn't very popular on the internet. But the more I use it, the more I like Steam as a way to buy games. I'm currently download Empire: Total War from Steam, I got Call of Duty 2 from Steam, plus a couple of casual and indie games. Without Steam I never would have found the strange wonder that is Dangerous High School Girls in Trouble! (and no, that is not the title of a porn movie). Now I'm not saying Steam is perfect, for example the download speed could be higher (I'm reaching 200 KB/s on a 6 MB ADSL line). But the fact that I need to be online to download or activate some games doesn't bother me at all. Hey, I need to be online to play WoW too!
As copy protection systems go, this is as unobstrusive as it gets. And when I get my new computer next week, I can easily reinstall all the games I bought on Steam without having to hunt for the discs. As long as the only thing copy protection does is well, preventing copies, I'm okay with it.