After reading about su.pr I was interested in giving it a spin (especially since I need to be concentrating on abstract algebra, I needed to take it for a spin right away). They posted beta invite code to the stumbleupon twitter feed. Of the features mentioned in su.pr's initial press release, all seem to be functional with the notable exception of "seo friendly" links (aka 301 redirects) so that search engines count the links as going to your site and shorten on your domain. I haven't had a chance to try out any of the other features, like suggested posting times, as it seems like they are tailored to each user so it requires a bit of data first. The settings panel seems a bit buggy (I haven't managed to get it to add more than one site that I'm "promoting"), but that seems like an easy fix. Overall I'm not entirely sure what all the buzz was about, it seems kind of cool but lacks sufficient compelling features to convince me that its not a bad thing to use a url shortener.