Any program that is sucking thousands, maybe millions, of bits of files from over the internet is going to use up some resources. Usually I run Azureus overnight but disable it when I'm working on the PC. This usually works out pretty well for me since other people tend to have the same idea.
At least it doesn't have spyware. That was the biggest problem with Kazaa, but was overcome by downloading the spyware-stripped Kazaa Lite. Sherman Networks, the company that ran Kazaa is still going through all kinds of legal battles. They thought that by locating their company in an "undisclosed off-shore location" they could avoid US copyright law infringement. Apparently they were wrong.
Bit torrent is the new medium because it's much harder to track pieces of files than full files themselves.
After the fall of Kazaa I used a client called Shareaza, which sort of combined bit torrent and kazaa... by downloading single files but as pieces rather than a whole. They came to an unfortunate end as well.
There's a huge resource of file sharing info over at http://www.zeropaid.com
. That website can pretty much answer your questions about any file sharing service.