I tried using it seriously 5 years ago. Things might have changed since then, though. All I remember is that it was tremendously slow and the results were pretty much garbage. It's an interesting project, nonetheless.

EDIT: They now have an online demo that doesn't require any installation. After waiting about a minute for peers to connect, queries ran relatively "fast" each time. That's an improvement. But the results are still junk, IMO.

https://yacy.searchlab.eu/Status.html