It's brilliant. If every home system crawls the Internet, you get a relatively complete index that is unhindered by censorship and bias.
Check it out here (http://yacy.net/).
Unfortunately, I had a bad experience with it. I think the software is perhaps not mature enough to be considered secure. Shortly after starting the YaCy client, Security Essentials caught three variants of PHP/C99shell. These were temp files named zipParser_xxxxx.php and zipParser is a Java thing (YaCy is written in Java).
I'm a big proponent of this sort of thinking though. Peer-to-peer technology is also presently used by Blizzard to distribute the SC2 client and patches.