I generally use !g if I want local results (say, looking for a restaurant) or I use !m if I want a map (which is really the same thing as local for me). I also use Google if I know my particular search is going to be obscure and/or noisy. For instance, weird error messages get a !g. The other major exception is that Google Scholar is incomparable for finding academic papers on a particular topic. However, for everything else (general knowledge, non-weird errors, most programming-related topics) I find the DDG results entirely sufficient. In fact, I intentionally use DDG by itself if I expect to end up at Wikipedia since DDG adds possible Wikipedia matches at the top of the search results.
That being said, I end up using the DDG results for >50% of my searches. If you do more local searches, or if the things you search for tend to be noisy or obscure then, of course, YMMV.
About the same as me! I generally do DDG first unless I know I want a map, or I'm searching for error messages. Other commands I use fairly frequently are
!yt (youtube)
!gmail
!news
!gg (google groups)
!hn
!boingboing
!torrent
!itunes
!rubydoc
!rubygems
!codex
!so (stack)
!am (amazon)
!jstor (and a few other specific academic journal searches, but you can also use !googlescholar)
!flickr
and probably a few others! I encourage everyone to check out the list and get it into their workflow. With Alfred + DDG I can get to most things much quicker than I could through google
That being said, I end up using the DDG results for >50% of my searches. If you do more local searches, or if the things you search for tend to be noisy or obscure then, of course, YMMV.