Shame its for sale by Green Man
What's the problem with Green Man Gaming? Something we should know about?
Every game I've ever gotten from them has required Steam to activate, i.e, they not only all work with Steam as if you bought off Steam, but in fact you need to use Steam.
I can also confirm that I bought the core game for 10.88, and it activated fine on Steam.
Slight problem though - it's a 30GB download. Woah. So I haven't actually started playing it, nor am I likely to do so until I finish some of the 80 other I need to play and delete so I can make some space.