I like DDG, and use it, but it's mostly a pretty front end on a now-deprecated Bing API. If Bing shut down, DDG wouldn't be far behind.

I've never seen a DDG crawler in any of my logs, though I've seen plenty of Google and Bing (and others). DDG does some nice instant answer and bang stuff, but the actual deep search results are all Bing (and maybe Yandex?)