It's great for comparison shopping, and very handy for finding surprising bargains. For example, the recent region 2 Breakfast Club dvd is only £6.99 from HMV, and over a tenner anywhere else : see? The listings include postage costs in their calculations, and covers both region 1 and 2 dvds.
Warning : Be prepared to spend ages searching on here as you think of more and more stuff you've always wanted....