I use DDG as my primary search now. Have been for months. It's getting a lot better. I'd say it's as good as Google about 95% of the time now. For about 5% of queries I end up bumping over to Google, but over time that's shrunk from ~15% of queries.
I've been using DDG for over a year now, and I haven't had to bump over to Google for a long time. In fact with the bangs I search pretty much everything with it.
I do wish the "defaults" were more trackless oriented though, for example, !i searches images... on google images.
Hmm... hard to say, but the first thing to comes to mind is programming-oriented stuff. Google tends to find a more accurate hit there sometimes. Google also seems to do a better job filtering out spammy stuff like those irritating portal pottie sites that clone StackOverflow or mailing list content.
My pattern is if DDG finds nothing of value or brings up too much noise, hop over to Google and sometimes it does better. If I can't find it via either search engine it's probably not out there.
I will say that there's rarely a case where Google is worse than DDG... unless you count the tracking and near-monopoly issues but those are peripheral to the core function.
