Good post Mark. Mixx is great. Everytime I forget about them I get reminded again. But funnily enough even though StumbleUpon is nofollow, they still generate significant traffic if follow your StumbleUpon guide. Great idea in creating this series, it's great.
StumbleUpon can def generate a ton of traffic. The only caveat with SU is the fact that you are driving traffic that is going to have a high bounce rate and low time on site. So they are not necessarily going to be as engaged with your content, as other referral sources. But nevertheless, an awesome traffic source.