Are you a fan of RTS games such as Command & Conquer or Supreme Commander?
It's one of those!
It's absolutely nothing like the table-top game at all. I was extremely dissapointed by this to be honest. I was hoping for something far more strategic, but they are pretty much Build as much as you can, as fast as you can, and charge. For every race. I find it dull very quickly.
Unless you're really into the 'Law and style of 40K' and you like those sorts of fast RTS games I'd avoid it.
But that's just me, some - obviously - love them!
I've not played the "Space Marine" game though. I can't comment on that.