I also use DuckDuckGo, and will likely continue to do so. I have it set as default on my home machines and my iPhone.
Apple's search so far appears to be related to the iPhone/iPad Home screen-based search and nothing more than that, although it is highly likely that it will eventually find its way into Safari. If there is no tracking/ad-based model then please, bring it on!
I suspect it will be inferior to pretty much every other search engine for some time, but considering what I use search for it will likely suffice. It is not Apple's strength, but it is something they can slowly build into (like Siri) while we rely on other, more robust search engines if Apple Search® is unable to find a good answer.
I see it this way: When Safari came out, I started using it right away and have never looked back. Same with iTunes, Pages, Address Book, Mail, etc. Got no use for "free" services from other companies, because I do not want to be their product any more than I already am.