Generally people coming from digg are real visitors looking at your site because they read something that interested them. I dont see why it wouldnt be valid traffic.
Not on my part... Most of the digg visitors open just the digged page, and don't even read it much. They are looking for funny pictures
Digg isn't usually good for adsense earnings because most on Digg are very internet / computer conscience along with having their own websites. Digg is good though for targeted traffic.
It is indeed not a problem to use adsense and digg. From my own experience I have to tell you that you won't be happy with the results, don't expect much clicks. This traffic is good only for alexa ranking.
Good as in allowed by google? Yes. Good as in good for CTR? I would say no from past experience. But I'm sure there are also others that had a positive experience with digg traffic.
CTR from digg- not so good... But traffic is great... Targeted traffic is even better... It helps your CPM ads from adsense very much! I get more money per click, because of my digg traffic... Also it is great for selling ads to people, because they know the traffic is targeted nicely
There are many peoples who use Digg as their main traffics and end as banned *or* got smart priced. Simple point of view is most of Digg traffics are not come consistency and not converted much so smart price would be applied over entirely your account and take a long time to regain account status back. Quality traffics from organic search results may be the best option.