Invites are so bloody rare. Gmail was groundbreaking (and empirically offered way more space, etc) so people were going nuts for invites. This is, at best, a "bit better than Facebook". And much as people dislike it, Facebook is a pretty powerful tool, as opposed to webmail before Gmail, which majorly sucked.
You could actually get in if you refreshed the main page, they seemed to let people sign up through some magical formula.