After seeing them pop up here once or twice quite recently I thought I'd give it a go, I opened up preferences in Chrome and switched my default search over to DDG. It felt good, made a nice change but was certainly a bit "odd". I found what I wanted, most of the time albeit it with a slight drop in quality vs Google.
But my biggest gripe with them was confidence. I'd just started out at a new job as a programmer and as you can imagine, I was searching for a good few things, usually things I was struggling with and I just didn't feel sure that I was getting the best results I possibly could be. You might think thats totally crazy, but when your battling an issue as many of you know, you want more than anything to work out whats going on. I didn't have any margin for error, I wanted the best results right there that second.
Specifics aside, knowing that Google is far superior in their results makes it real difficult to use another search engine really, extra features (!so etc) or no extra features. I personally search because I need to "find" something and I usually don't know where that something is, opening up Stack Overflow isn't that much of a chore for me. Its the other bit I need help with.
Anyway, I commend their mission and hope they succeed in taking a fair slice of the market. I think he's a great entrepreneur and I wish him the best of luck. I can't imagine what it must be like to be head on with... Google.