Private Beta (250 Invites) - Nice touch to hype up a URL shortening service by creating an illusion of scarcity.
I used the private beta to take a look inside.
On the surface it looks just like another URL shortening service. While its track back feature does get one excited, but then is it any better than bit.ly or cl.igs
StumbleUpon users may have a different view, but then I am not a StumbleUpon user.