I work at a Blockbuster store and every week we have movies that come in with just a few copies, these are the kind of movies that the Sci-Fi channel shows. The kind of movie that nobody ever wants, and only that idiots rent, when they bring it back I ask them "was it any good?", they say "no we turned it off after 15 minutes!" Movies with terrible computer generated, super imposed monsters and such like, very unappealing.
This is the same type of movie that Grendel is, and absolute waste of time, if you want a reasonably (and only reasonably) good Beowulf based movie then try Beowulf & Grendel , starring Gerard Butler, who is also starring in the eagerly anticipated 300, as King Leonidas of Sparta.
Plus, later this year we have another Beowulf movie, with a star studded cast ranging from Anthony Hopkins and Brendan Gleeson, to Angelina Jolie and John Malkovich.
But don't let that get your hopes up like we all did with Eragon, or we are all in for another big disappointment.
And regarding rentals, here is my rule of thumb: If there is only one or two copies, don't rent it because its a load of crap.( This is true 99.9% of the time, usually not true if the title is foreign, or a documentary.)