On paper the G5 should have good performance. In practice, in the hands of Mac OS X, it never seemed to acheive its full potential. AnandTech think they've found the culprit: Mac OS X. Here are some quotes:
"The G5 might not be the fastest integer CPU on earth - its database performance is completely crippled by an asthmatic operating system that needs up to 5 times more time to handle and create threads"
"The server performance of the Apple platform is, however, catastrophic."
Interestingly, Linus Torvalds, in his book Just for Fun, says pretty much the same thing (he describes the Mach kernel at the heart of Mac OS X as "a piece of crap"). His critisims are exactly reproduced in the above AnandTech article - I had assumed he was spinning a "my OS is better than your OS" line.
Of course, Apple are not really about performance. They make stylish, user-friendly computers for people who don't like to be part of the herd. Furthermore, the above analysis only really applies to data intensive applications, so will not be relevant to standard desktop applications such as Microsoft Word.