The toolbar has "I like it" and "Not-for-me" review buttons to rate the site being viewed, using these ratings to find sites that people similar to you have liked. A "Stumble!" button sends you to one of these recommended sites.
StumbleUpon also provides a chance to do some egosurfing, even without the toolbar: Users write site reviews, which are shared publicly.
Each user's reviews and comments form a weblog with its own RSS 2.0 feed and another feed for comments. Here's one for Janah, a user who enjoys cats, gardening, Led Zeppelin, Robert Heinlein, and me me me!
The end result looks like the out-of-wedlock love child of LiveJournal and de.licio.us. I can't decide if or the site's , but you could do cool things with that data if the developers offered an API.
A couple of years ago, I installed the IE version of StumbleUpon, and it behaved badly - spyware type symptoms like popups were the main annoyance. I cannot say scientifically that these were caused by SU and not some coincidentally installed malware, but I still believe it was SU. SU also proved to be VERY difficult to uninstall at that time. I was too pickled to remember specifics, sorry.
I installed StumbleUpon more than a year back on Firefox. No spyware at all. No adware too. Heck! There are no ads anywhere on the site or the StumbleUpon pages. It's supported solely by the members who choose to pay so they could be "premium" members.
It's a very effective way to waste time though and I've spent hours I've never going to get back.........