Having read Paul's post yesterday, I instantly applied for a beta invite. In fact, while writing this blog post I'm listening to a very pleasant stream of music from the echotron. I'm impressed! It couldn't be much easier to get started. It's very easy to quickly add a bunch of artists I like to my profile, and I even get to listen to any search results. The recommendations are pretty good, definitely a lot better than many others I've seen out there... and they are sometimes rather different from the ones I get at Last.fm (in a good way). However, despite being biased, I'd still argue that Last.fm's recommendations are better ;-)
Anyway, it seems the echotron could easily turn into more than just a site built to showcase the echo nest's APIs.