Kazaa is full of malware and spyware.
In eMule you can usualy get a reasonable quality files.
If you know specific things you do want, I would suggest using BitTorrent and use MiniNova, a BitTorrent search engine to look for the stuff you need. Pick the file with the most active users (leechers) and seeders.
BitTorrent is based on downloading a small file to your machine and give it to your BitTorrent client. In most cases if your BitTorrent client is installed it will be associated with .torrent files, so you can just click "Open" when you download the file instead of save.
There are a lot of BitTorrent clients. I personally use uTorrent (http://www.utorrent.com) which is a very small, slick and fast BitTorrent client.
If you want to search just music files, try using SoulSeek (http://www.slsknet.org) which is also a P2P file sharing application mainly targeted for music files.
DISCLAIMER: Do know that downloading licenses content without buying it is illegal in some places around the world.