Newsvine for me was the most useful site for credible news recommendations. Digg and Mixx were more of a People and US magazine version of news - fun, flashy but not necessary sources of substance. Reddit offered up some interesting news bits and seemed more middle of the road for me as a source.
I find that I will prefer Newsvine over the Yahoo and Google news engines. I've used Google and Yahoo surfing for news to read because they're easy. (I like easy.) But they are very limited in their presentation and I guess I'm ready to graduate to a better level of news service. ((I know you'd think being in a library I'd read the newspapers we get but I've seen people read the papers who use other things than Kleenexes to rid objects from their nose and then go back to flipping through the paper. No thanks.))
Mixx and Digg leaned more towards being productivity drainers. Nothing wrong with that if that's what your looking for.
The article that I tried to add to Newsvine - I submitted it but it didn't show right away. I wonder if that's normal for a new submission??? The second article I tried to submit was already submitted - as it was a week old.
I'd really be interested in the demographics of the members of these groups. Just for curiousity's sake.