It has greatly improved over the years. I remember it being unusably bad when I first tried it in 2013, but today I only have to revert to Google once in a while. That being said, there are rare occasions when it fails spectacularly, bringing up totally irrelevant results for simple searches.

It seems to expect more precise queries, while Google is geared toward "close enough" searches. I've found that DDG is often preferable when I have an exact phrase in mind. Google is sometimes too helpful, correcting errors that are not actually errors and failing to take some queries literally enough.

The main issues I have with DDG are that it doesn't seem to prioritize recent results (which can be good, but usually isn't) and it fails at local results, which is basically by design.