I am very active with StumbleUpon and my reach is very broad. Technically, I am not SEO/SEM but I do employ these techniques when building a site for a customer or managing one of my own sites.

Lately, I've been doing more reviews and I am being selective who I add as a friend. If you are active with SU, that is good. If you aren't, even if you are an A-Lister, then you have no priority over any other stumbler.

I agree with Lyndon: There doesn't seem to be any extra juice given by someone who is considered a power stumbler. Everyone seems to gravitate to people with a lot of stumbles, fans, and reviews but even the novice stumbler can benefit by being connected.

http://mattkeegan.stumbleupon.com/about/