It's a Halo game without Bungie! It's a real-time strategy game on a console! Halo Wars is all kinds of unexpected. So with a demo now out, feel to try before you buy/vomit opinions.
It's well over 1GB, and while at time of writing there were still a few kinks in downloading it (namely, despite Microsoft saying it was there, and some people being able to grab it, I couldn't actually download it), by the time you read this it should hopefully be all plain sailing.
Demo: Halo Wars [Microsoft]