I have 3 sites.
I am testing stumbleupon since three months and the good thing is I am getting 20% of my daily visitors from stumbleupon. It is really really easy way to get traffic. I recommend everyone to use this. You can do exchange stumbleupon bookmarking very easily using
stumbleXchange :: Exchange Stumbles to Promote Your Website you can get people to exchange stumbleupon bookmarks in this website.