This is only anonymous in a casual sense. The relaying party knows who both of you are. For anonymous email in the cryptographic sense, see http://mixminion.net/.
This type of site has value for the low-risk situations where you don't want to deal with the high latency of remailer networks. You trade all but the most rudimentary security for that convenience. And in a lot of situations that is fine (like the lock on my front door).
It looks interesting, but I think you're going to run into a trust issue.
If I wanted to email someone anonymously, I'd feel more comfortable just going and registering a free email account with gmail or hotmail or something. Then I don't have to worry about what happens to the data on your servers.
What's the argument for using this over just registering an anonymous account somewhere?
This type of site has value for the low-risk situations where you don't want to deal with the high latency of remailer networks. You trade all but the most rudimentary security for that convenience. And in a lot of situations that is fine (like the lock on my front door).