I've been using DDG at home and at work for at least a year and boy as there been improvements! First of all, I'm swedish and queries in swedish have improved immensly! I can't remember DDG ever being notably slow, so I don't know for the speed-up (I don't care much for speed). But answers in english have improved in quality by magnitude of 10s. When I started using DDG I'd have to check back with google about 1/3 of my queries, but now it's around 1/50, and I can't say the way I do my searches has had any radical changes either.

Thanks for this