I got a sub $500 Dawes from Bikesdirect that is my main commuter and it rocks. if I were buying a pure commuter I would go with wider tires but i got it for other things and it just happens to work really well.
If you can do your own mechanics work, buying used will general get you more for your money. If you want a new bike with a (however limited) warranty with nothing that needs to be replaced or repaired and wont start making some funny noises a month after you buy it ... BikesDirect is a very good source.
I notice that a lot of people are rabidly opposed to BikesDirect ... and none of them have ever bought a bike there.