Infestation really is a great game, though it is a fairly short one. I've played through it all on the Wii version, and while it is an enjoyable (albeit easy) ride, it only lasts about 5-8 hours tops. The co-op mode extends the game play significantly if that is an option. For the Wii, I would highly recommend using a classic controller for true twin stick shooter controls.
Also, I would recommend checking out your local Gamestop, used copies go for $4.99 now (Wii Version, $6.99 for the 3DS), and the ones I've seen are all in pretty good (near new) shape.