If you have a specific niche it's great, but if you are trying to make money in general it will not convert well at all.
You can definitely use SU to drive traffic and improve alexa rank. As for as monetizing your site is concerned, the SU visitors are poor readers and have the least chance to click the ads in your site.
Waste of time because most people are simply clicking the "stumble" to browse because they are bored not to buy anything.
StumbleUpon is great if you use it correctly! No one using stumble wants to buy your crap so don't try selling it to them. Instead, give them great content targeted to their interests. SU has tons of categories ranging from jazz to homebrewing to kinesiology. Give people good content in a specific category and they will give you a thumbs up and you'll also get repeat visitors. Also be aware that many SU users are also webmasters/bloggers/publishers. If your site/article is good enough you will get backlinks from people who found you on SU. So quit using it for sales pages and give people what they want!
Talking from experience, I usually notice a huge spike (2-3K uniques) on the day I stumble it. Afterwards, I get 0 traffic. Stumbleupon sucks for backlinks as they are nofollow (perhaphs only for the anchor text if that is of any value for Google SERPS )>