i like it. don't make as many sales as i would like off it but it's great for brand awareness and excellent for seo as every social networking sites are
I've sold a few copies of my software to visitors from SU, but I figured I just got lucky. It's nothing I've ever tried to push from a marketing angle since it seems like the kind of thing people do when they're bored - not when they're looking to buy something.
With StumbleUpon, you need to learn the system in order to benefit personally. This means adding friends, stumbling their pages, stumbling categories, carefully selecting pages for inclusion into the StumbleUpon system, stumbling on a regular basis. You may not get thousands of hits, but you should see a regular number of stumbles come your way.
Through tracking: I kinnda figured that StumbleUpon users DO NOT tend to revisit your site again in future. They're only interested in the thing that was interested in first place. As soon as it's been seen/experienced, they're otta there!
The StumbleUpon users don't like AdSense too. But sites about wallpapers, games, jokes, etc. could get a lot of traffic from StumbleUpon
Traffic from SU is good. People stumble your site, and add it to their profile which counts as backlinks to your site. They might also blog about it if they find it useful.
This month I have received 1,127 hits from stumble. All I was was participate in as many stumble exchanges as I could find
I have quite a number of sites via stumbling I would not have otherwise found had I not used this tool. I have bookmarked several of them, returned to add comments (if a blog), and in many cases have added these people as friends.