just traffic. Most people dont link to you because of it, but you can get a heap of traffic in a short period of time.
this would have been a good thread to put a poll on. I don't use digg much. I stumble, but I just use it for traffic.
I'm still experimenting with this- but you could use them to get traffic initially, and then send them to a page where you have useful free widgets or scripts etc, with links back to your site. So you would get traffic initially, but build links long term...
Stumble is purely for traffic (you could easily manipulate alexa rankings that way ) For digg, it's for exposure/marketing. I don't think backlinks would be valued that much from the digg site.
Google also advise to build link and reputation through digg.com/yahoo answer. it is good when trusted websites link to you and help you in traffic. http://googlewebmastercentral.blogspot.com/2006/12/building-link-based-popularity.html
digg and stumble traffic only stay for up to 30 seconds then leave so what is the point and doing so? Maybe if you got on the homepage of digg would be pretty cool but other than that whats the point in trying? I rather for my readers to digg and stumble my pages without me asking.