It's not a well written movie. It's got a ton of "duh" moments in it. If you're a fan of either of these franchises then you'll probably get annoyed with the changes to the lore, especially the Aliens. The human team up with the Predator is pretty damn dumb.
That being said, there's a few cool Alien Vs Predator fights, some cool special effects, a penguin, Lance Henriksen, a Queen Alien, bad ass nets and other weapons, snow, spiffy space ships, and quite a lot of shut-my-brain-off-and-enjoy fun. If you think about this movie at all while watching it, you'll probably end up hating 90% of it.