I am running OS X on three different machines. Below from slowest to fastest is the rundown. Overall I find OS X exhilerating to use, in fact, I can't stand having to reboot into 9.1 to use certain Apps and try and get out of them as quickly as possible to come back to X.
G3/266, 192MB RAM, 20GB ATA HDD (this is an add in drive to replace the 6GB that came with the system - it is an ATA 100 running at ATA 33 off the system board), and stock ATI video: This machine is by rights the slowest i have running OS X. It is also at the very bottom of Apple's support list. All-inall, for what i have running on the system it is not bad. The window dragging and Finder are indeed fairly slow, but not unusable. According to macosxrumors.com, however, the upcoming update will enhance the performance of the OS on G3 systems more than G4s. As well, I am going to patiently wait for a revamp of Quartz so that it takes advantage of the GPU instead of the CPU as it does now. I am not using any non-native Apps on this machine (e.g., no Classic)
G3/400 B&W, 256MB RAM, 20GB ATA 100 HDD (running off the system board at 33 or 66 not sure), and stock ATI video: No ifs, ands, or buts, about it. This system is highly usable and fairly fast. I currently run several Classic apps, including Photoshop, InDesign, and Illustrator, all of which run faster in classic then they do booted into 9.1 (I have done speed tests that verify this, including batch processing using PhotoShop actions). There is still some need to go down to 9.1, but nothing excessive. And, the multitasking capabilities are awesome.
G3/400 PowerBook, 192MB RAM, 10GB HDD, and stock ATI video: Almost as fast as it's desktop sibling. The biggest hassles are window resizing and the Finder in list view (column view is pretty fluid). No Classic Apps for this set-up. It is basically my test bed for new software that has been ported over, that I have never had the opportunity to use before (i.e., MySQL, Python, PHP, etc.). I wait for the first upgrade with baited breath to see if I get any real speed bumps on this machine.
G4/500 Dual Processor, 768MB RAM, 40GB HDD, ATI Rdeaon 32MB OEM video, Apple Cinema Display: Can we jst say... SCREAMS!!! Yes, it is that fast. A little sluggish on window resizing and in the Finder, but still faster than the P3/800 and Win2k box that I use for Firewall maintenance. i run a lot of Classsic apps on this box (i.e., PhotoShop, Illustrator, Flash, Director, GoLive, Office 2001, Peak, and more). I do need to reboot in 9.1 to use GoLive efficiently, but everything else leaves scorch marks on anything I've done in 9.1 or earlier. The system is also fast in native OS X. Quartz being completely CPU driven just gives me a fun reason to watch the CPU Monitor and see my second processor get used. I've also spent more time tweaking this machine from the command line, and getting it to do what I want from a workflow perspective. Biggest benefit to this system is that running OS X allows me to use tools I never thought I could run on a MAc system, once I test them on the PowerBook I move them over to this machine andjust have a blast. I'm even thinking about investing in Lightwave 6.5 when it is released, it should be nice to set a reder to process in the background while I listenm to MP3s on iTunes, scrub the dock looking for like three different browsers, play a game of Down&Out and run a few FTP downloads at the same time.
All-in-all I can say this, OS X is not your mama's Mac. it never will be... hopefully. But, if you give it time and actually try and get used to all the new features and ways of doing things, you'll probably end up loving it. And, i have faith that we will see a consistent stream of optimization updates from Apple, as well as continually being notified of more and more commercial apps going native. Not to mention the string of announcements about previously unheard of applications running on our Macs.