Wishbone: So which is it? Is it "Log on if you want to play multiplayer" or is it "Log on if you want to play at all"? There's rather a big difference, you know.
amok: you need to log into the launcher if you want to use it, but it can be by-passed by starting the exe directly. When the game runs, you have the option to log into your runic account. If you do not do so, you can only play SP, if you want to play MP you need to log into the account.
So, you do not need to log in at all if all you want is to play SP. However you need to log into something if you want to fetch the latest patches or if you want to play MP.
I don't have to login to the launcher though. The launcher updates the game without login. When I start the game from the launcher for the very first time after installation I get a prompt to enter and verify the CD-key which is anonymous. After the key is verified the game does not need online capabilities, not even the launcher. Multiplayer access and launcher updates need to be online for obvious reasons of course and multiplayer only also needs an Runic account.
I think it is quite an unintrusive DRM, apart from the activation limits a key has, but as Tiny mentioned these should be able to get revoked easily by asking Runic. Better than Steam anyway because it is anonymous, does not need a separate client and does not need an account for single player.