Not buying a game because it doesn't have a demo is one of the dumber things I've heard in a while. A demo is often a cut-up version of a game, with many features missing or altered. A good game review is worth more than a demo. There are dozens of sites that offer free reviews (just check this site on a daily basis). Just check a few out and find one with reviewers that you agree with, and see what they have to say about the game. All good reviews are based upon full versions of the game, that has been played through to the end (hopefully even multiple times) by the reviewer. That info is much more valuable than anything you can discern from a stunted demo.
And by the way, there is no way DOOM III is putting anything in a rut. By the time it comes out, Unreal II will have sold millions of copies, ensuring the success of its parent companies and more than likely spawning multiple sequels.