yes ... its very useful to drive traffic and i would still prefer to use Digg for more traffic even if its nofollow now.
Digg is the most powerful social site out there, so many times when I tried to digg sth 9 times out of 10 someone else has dugg it already, never happened on other social sites before. Good for traffic even nofollow.
Digg comments were nofollow for quite long. Digg is helpful in generating good traffic and SEO is a value added service.