If you make an intriguing site that the random user might find worthwhile or entertaining then the traffic from SU can be helpful. For most marketers however it's not worth it.
Here's my view: Stumbleupon can be good for traffic, but it could also be harmful because if a user is totally p****d at your website, they write a bad review. As for nofollow, it's traffic. Any traffic sources are good but I personally wouldn't spend all of my time on it. Time management is important. Spend a bit driving direct traffic to your website, a bit link building and more importantly a bit of time updating. I have social bookmarking links on my website. I haven't and won't go to these websites submitting, the users will do this for me.