I spent about $200 on it promoting http://www.christmasbingocards.com last December 1st through December 17th or so. The idea for that site is it pitches exactly ONE thing with laser focus. The idea for the advertising campaign was "Get this in front of technically aware moms, have them blog about it, win".
I received overwhelmingly targeted traffic. They loved it. I got plenty of thumbs. 95% of them left within 2 seconds. The remainder did not give me the blog links I was hoping to get from the experience.
I got much, much better results from the concurrent campaign I did on AdWords. User engagement was much higher (downloads, comments, clickthroughs to my site, time on page, etc). If you compared prices per interested user (defined as someone who didn't bounce in 2 seconds or less) AdWords was something like a twentieth the price of SU. And I wasn't even using it for conversion related purposes -- if I had, I think I'd probably have spent about $10,000 on SU for every $25 sale I made.
I will not be using SU again for this site this Christmas, or for any of my similar promotions. It is just drastically inferior to my other options for spending time and money to promote things.
(Incidentally: I received a few thousand organic stumbles, too, and they were about as useless as the ones I paid for.)
I'd like to qualify this with some additional advice.
Advertising, on SU, a site that's not build for the SU audience, is likely to fail exactly as you've described.
If you're going to advertise on SU, you should craft the page that you do advertise to be instantly attention-grabbing, short, and tuned to give you whatever result it is that you want.
http://www.christmasbingocards.com/ , for example, is clear and immediately obvious, but not very attention grabbing. If I'm stumbling around looking for interesting stuff, I will not be grabbed by this page, no matter how well targeted to my demographics it might be, because it's not targeted to my frame of mind.
On the other hand, if you'd published a cartoon instead, for example, something obviously funny, chances are most stumblers would have read it. Then, your job would have been to create a cartoon that's funny and also encourages the reader to click around the site.
That's no easy feat, I'll grant you, but the point is, if you want to get useful traffic out of StumbleUpon, you have to tailor the content to the channel-surfing frame of mind.
I received overwhelmingly targeted traffic. They loved it. I got plenty of thumbs. 95% of them left within 2 seconds. The remainder did not give me the blog links I was hoping to get from the experience.
I got much, much better results from the concurrent campaign I did on AdWords. User engagement was much higher (downloads, comments, clickthroughs to my site, time on page, etc). If you compared prices per interested user (defined as someone who didn't bounce in 2 seconds or less) AdWords was something like a twentieth the price of SU. And I wasn't even using it for conversion related purposes -- if I had, I think I'd probably have spent about $10,000 on SU for every $25 sale I made.
I will not be using SU again for this site this Christmas, or for any of my similar promotions. It is just drastically inferior to my other options for spending time and money to promote things.
(Incidentally: I received a few thousand organic stumbles, too, and they were about as useless as the ones I paid for.)