well, i have not much exprience with bookmarking systems but i can assume that traffic will be targted.
Depends by what you mean by 'good'. If you mean 'quality' then maybe. Some sites are better than others. If you mean 'quantity', then not so much unless your site becomes very popular (i.e. lots of diggs, shouts, saves, stumbles, etc.) Get on the front page of any social site, then your website could be overwelmed.
I think traffic is usually good except you usually won't any ad clicks .. they usually read and leave the site .. at least from a study done by some pro bloggers..