Downloadable games designed to promote upcoming titles are all the rage these days, but I can't think of any that are actually good. Maybe Case Zero for Dead Rising 2? In any case, Warhammer 40,000: Kill Team isn't exactly what I would call "bad," but it's not really what I would call "good," either.
Basically, it's a twin-stick shooter in the vein of Housemarque's Dead Nation, but with Space Marines -- not an unappealing prospect by itself. There are four classes, and with the possible exception of the Assault Marine (the Jump Pack super is a little too inaccurate), they are all useful and interesting in their own right. That's really half the battle right there, since it encourages you to replay levels with different characters.
And you'll want to try out different classes if you want to get your money's worth out of this game, because the campaign is exceedingly short. There are five levels, with one of those levels dedicated to taking out an Ork Warboss. Beyond that, there are a handful of Survival levels, which are enjoyable if you want to shoot for a high score or two on the leaderboards.
I'm not hugely bothered by the length though -- just the execution. I think I might have actually preferred a fixed isometric perspective along the lines of Death Nation, rather than the constantly shifting perspective of Kill Team. It's disorienting, and it really doesn't do the simplistic graphics any favours when the camera zooms into the Space Marines.
It's particularly bad during the final boss battle, as the angles used are both awkward and make the Space Marines seem tiny.
Also irritating is the slow motion effect that kicks in during even comparatively minor kills. Like the shifting perspective, it's both pointless and disorienting. Add in the occasional gaps that are almost the same colour as the walkways, and Space Marine can be as annoying as it is entertaining.
In this case, I think simpler would have been better. Stripping out some of the needless camera angles and the slow-mo would make it to focus on Kill Team's strengths, like the fact that it's impressively fun to annihilate a horde of Orks with a rocket launcher. As it is, Kill Team is a reasonably fun promotional vehicle for Space Marine, but a sloppy one.