Honestly I think StumbleUpon traffic is pretty much trash. If you want to have a serious business I would rather have 1 legitimate person who was interested in my service/product than 100 people from StumbleUpon who have absolutely zero interest and are trading off time for other StumbleUpon people who really just want their traffic stats bumped up.
I've never heard of anyone receiving a sale as a direct result from the traffic you get there. That doesn't mean I wouldn't throw my site into the mix, it's just once I submitted my site, I wouldn't bother messing with it after that.