Yes, you can customize Chrome's search box in "Manage Search Engines". I've setup a few frequent searches -- w=wikipedia, am=amazon, anything-you-want for site:anysite etc.
That feature is in almost every browser, and has been for a while. And it's great if you only use one computer all the time.
But I have my phone, my iPad, my personal desktop, my personal laptop, my work desktop, etc. Using the browser's custom search feature means I have to add custom search engines to every browser on every machine I use, OR I have to setup syncing between them all, which I really don't want to do, and isn't even really possible between e.g. Safari on iPhone and Opera on Linux. And even if that were easy, I really don't want to sync browsers on my personal machines with my work machines. It's a lot easier to just set the search engine to DDG, or go to the website and use !whatever from there.
I've been using DDG for a few years now, and I've been very happy with the search results. The biggest weak spot, IMO, is that they don't index as many old mailing lists and forums as Google. The only times I ever need Google are when I'm looking for very specific or obscure error messages which aren't always covered in regular documentation, but tend to be asked about on mailing lists.
That said, it wouldn't surprise me at all if the quality of results depends a lot on how the search is phrased and the topic being searched for, etc., so I could just be getting lucky.
This is a killer feature for me as well. Unfortunately DDG's results for other things lack compared to Google, like searching for specific technical things like "how do I do this in code" or "wtf computer why are you doing this specific thing". Google's also a lot more helpful with forum threads in their results.