The Game Collection are currently selling the not-too-old Aliens vs. Predator for a pretty bargainous price of £12.99 for the XBox 360 and £9.95 for PCs (unfortunately the PS3 version is currently unavailable). For both versions, this undercuts the next best price by about £2. Elsewhere, you're looking to pay at least £11.97 (from Amazon) for the PC version and £14.85 (from Simply Games) for the 360.
Aliens Versus Predator is almost like three games in one as it is made up of three separate campaigns, each comprised of five missions, giving you the opportunity to jump into the shoes of each of the factions: Colonial Marines, Predator and Aliens. You get an individual story for each campaign but they also fit together to form an overall narrative. As a predator you must travel through the jungles of your home world as you take on the rites of passage that allow you to change from "young blood" status to "Elite" before you join a group of your pals to investigate the Case of the Missing Young Bloods from a neighbouring planet. As an alien your task is to escape the laboratory that you have been trapped in by its resident horrid scientists before making your way to the marine's colony to have destructive fun with your face-hugging pals.
The Alien and Predator segments offer some unique(ish) gameplay: as an Alien you can wiggle and scurry your way up walls and across ceilings while Predators can fling themselves around, pinging off surfaces in their bloodlust fury. Sadly the awkward control system means that the novelty of zooming around in Alien or Predator form quickly wears off and the game can become rather tiresome. Meanwhile, the marines' campaign evokes a decent tense atmosphere with some genuinely creepy moments but, for the most part, it is just a fairly standard first person shooter.
Aliens Vs Predator is certainly a lot more fun than I was expecting, and with the three separate campaigns to tackle, offers good value for money (especially at this price) but don't go expecting anything mind blowing.