I've been a fan of StumbleUpon for almost 6 months, and here's why:

The above image shows the traffic stats from the StumbleUpon toolbar to SEOmoz since the beginning of the year. While these numbers are by no means overwhelmingly large, my impressions of StumbleUpon is that the traffic is fantasticaly well-targeted, savvy and looking for viral content. Stumblers tend to have accounts at Digg, Del.icio.us, Reddit and others and more than once, I've seen Stumble traffic lead to a re-surgence of traffic to a particular site or page that Stumblers have given high marks.

To be honest, I'm also a user of the toolbar. I have it installed on my laptop, home machine and work computer and will occasionally "stumble" while chatting on the phone, waiting for a process or when I feel burned out. It may seem counterintuitive to use the web for random navigation - something that only Stumbleupon offers - but to me, it's relaxing, fun and exposes me to content I'd never see on my own.