I'll be honest with you guys. I bought the DIR-657 and I can't notice any difference in speed or performance over my 4500. Like Harry (and most of you) the nerd in me wanted to try it out. It's got some quirks, one of which I found last night after about 2 weeks of good, reliable performance. It lost internet connection although I could still connect and fiddle with the UI, thank goodness.
Ultimately, I had to do a factory reset and used the setup CD to configure everything. So far it's been stable again. Initially, the setup CD would hang on the last step and give me an error. This time, it worked all the way through.
That being said, I think what Harry said about the 4500 being known to have now....dare I say....incredibly stable firmware and a wealth of guys here who know it inside and out, and it really makes a difference when you have people who can help you configure your router and tweak it to your needs. One can have a state-of-the-art, new router but if it's not configured right, it can be unstable and quirky as any other.