I tried Blekko for a bit this past fall and was not terribly impressed. I, frankly, don't understand what the /slash notation is doing. is it a filter? Is it a specific type of search?
However, I have been using DuckDuckGo almost exclusively for the past 2 months and I'm not sure that I'll be going back anytime soon. DDG is touted as being for the privacy conscious, but I can't say that was my primary motivation in switching. I have been increasingly unhappy with Google's 'bubbling' and my searches there increasingly felt like I was running in circles through crappy mailing list archives and spam sites and all their new 'features.'
DDG brought back memories of what drew me to Google in the first place, over a decade ago. It is clean, straightforward, and relevant. Yeah, I know it's running on Yahoo Search APIs which are running on Bing, but it's still a drastically different search experience.
Finally, doing a bit of research into DDGs extras and the !bang notation pages kind of sealed the deal for me. Want to know your IP address? just search "ip" Want lorem ipsum text, just search for that. Want to search the python documentation? Just use !python. Want to re-run your last search in Google? Just add !g Those features alone are enough to make it a true power user's search engine.