Jeff Atwood (from Coding Horror) ran a comparison of bzip2 on 8 cores against 7-Zip on 2 cores in February. bzip2 ran faster but generated larger files.

"The only reason to go bzip2 is if you need the file compressed as rapidly as possible to a reasonable size.

On pure efficiency / energy, 7zip wins, despite bzip2 using 4x the CPU time."

http://www.codinghorror.com/blog/archives/001231.html