I never really found StumbleUpon to be useful or even entertaining. I get enough new websites through my feed subscriptions everyday not to need random suggestions of where to spend 5 more minutes online.
Google though have hit on something a bit more interesting. The Google Toolbar now includes a dice icon that uses your Google search history to recommend a new site.
It will take more than a few minutes to see if it really works but basing recommendations on queries you have typed into Google in the past rather than votes by other users seems a better strategy.
