The thing that attracts me to the "StumbleUpon" site is not the random aspect of the "Stumble" button but the not-so-random feature of being able to visit the profiles of others who liked the same site as me.
These other stumblers have their recent picks and blog entries on the profile...
One can find clusters of stumblers with common interests who then will recommend more sites. Your own profile will list the other members who recommended the sites that you clicked as "great." There are subtle complexities to this system that are not at first apparent.
True, if all there was to the toolbar was the "stumble button" it would differ little from the almost infinite other varieties of surfing styles.
Perhaps the membership could grow some more to reach a critical mass for certain specific interests. Their membership has risen from 40k to 42,517 members in the few days I've been watching.
I do sense some momentum. But certainly not for everyone.