My problem with stumbleupon is that while you may get many visitors (in the thousands per page), I noticed that very few - if any - actually cared. Did some analysis on it and I wouldn't use SU again: http://ssdpress.com/when-it-comes-to-blogging-readership-its...
Good point. They don't stay long, and they likely won't sign up for anything. If you get a big spike from SU visits, it could take down the site. And you'll have nothing to show for it.