I think what people are experiencing here is the shift from OS 9's method of handling memory to OS X's method. In OS 9, a single application in the foreground could "hog" all the system resources and appear to be zipping along quite fast. In OS X, protected memory and multitasking won't let a single application do that.
For example, in OS 9, Final Cut Pro's export feature basically brought the system to a standstill and prevented the user from being able to do anything else in any other application until the export was done. In OS X this isn't the case -- you can happily switch to Safari and continue surfing the web while FCP exports, with only a modest speed hit.
I have Panther installed on my G4/400, and this is the Yikes! machine... Panther zips right along just like OS 9 did, but in a different way. I would have to say that OS 9's user interface was a little faster, but the productivity I have gained from being able to perform multiple activities at once under OS X more than makes up for the percieved speed hit.
This is only my opinion, but Panther runs just fine on my G4/400 (well, it HAS been upgraded to 500MHz, but it's still a PCI/Yikes! machine). Sure, I'd love some more speed -- who wouldn't? but I would hardly call it unacceptable. I use this machine for daily web/email tasks as well as some PhotoShop/Illustrator/Flash work and it's just fine.