Personally I've found that the extra features on top of Yahoo's results that DDG gives aren't usually very useful, but it's nice and fast with a simple, usable interface. For the few weeks I had it as my Firefox search box default, the only thing I felt was missing was how Google includes results from Google News and Image Search right on your search results page.
Also, the founder of DDG posts here on HN, which is one reason why you'll hear about it around here more often than other places.
Curious, what do you think the plethora of other alternatives are? I've been pretty unimpressed by the small search engines trying to use their own indexes (Cuill) and even less impressed by the "human-powered" engines (Mahalo). I haven't seen anyone else really come close to Google, Yahoo, Bing, and Ask.