I tried, years ago, I think their federated approach to building a database and crawling are cool - but at least the last time I checked, the actual search algorithm was just too bad. It often gave me completely irrelevant results and seemed very susceptible to spam sites gaming Search Engine Optimisation.

I’ve used Yacy for years. It’s very customizeable and gives good results, and I like that I can curate, to some degree, what it finds. Biggest drawback is that I don’t always feel like starting up a server to do a search.