What many may find useful are comparisons against A PowerMac Quad 2.5GHz system.  In his review, Bean states:

"While the PowerMac G5 Quad felt more like a Mac truck than a race car, the Mac Pro 2.66 GHz feels more like the race car. The startup time is more than twice as fast and it processes H.264 video with amazing speed. It could be argued that a few of the Mac OS X services are still starting up after the finder appears on the monitor and that this is just a speed trick. Even so, the Mac Pro feels like a considerable speed improvement over the G5 PPC. Using non-universal applications with the Mac Pro can change that feeling in a hurry. They take much longer to launch than when using the Quad. However, I see this as a short term issue. Within a year every application that you might want will be available in a universal version."

The detailed review covers the following areas:

A highly recommended read that can be found at the Arizona Macintosh Users Group site.