If done properly, parallex effect tends to wow every time - at least judging by my clients' reactions. I have just come across this article which neatly describes how to do it right. Worth a read.
The first parallax site I ever saw was Ian Coyle’s BetterWorld for Nike. I was in love. At the time, I’d never seen the technique before. It felt like I’d left the web of PDF-esque static pages and entered the future.