I've played with this site a few times over the last year. I just did various designs for fun. I did have some trouble drawing anything complex so that it could be machined, but that just might be me. If you actually do design a part, one of the keys to a good price is to order more than one. Essentially the first one is expensive (setup cost), but additional pieces seem reasonable. Also there was an article about eMachine in Wired magazine a few months ago. The author designed a custom guitar body made out of acrylic.
Another cool site for custom motorcycle parts is
mechwerks