As a developer I have kept a keen eye on DuckDuckGo for at least two years (note: I wrote a basic search engine in C# last year). Frankly, it’s amazing that one person has been able to create this search engine and I do wish the guy all the best with his venture. What amazes me is his success with what seems like fairly well-documented search engine techniques. As far as I am aware there is no uber-complex Mathematics behind it, nor is there no “Google-killer” algorithm. It’s a simple search engine by common practices that does its job incredibly well.
I’d say the results it gives are as good as Bing and Yahoo, better in some cases and worse in others. However, it’s still quite far behind Google in regards to search quality. I’d recommend people to use it for basic searches, but more often than not we want specific things and Google tends to find these very well.