So how does 24 measure up in the wild? After downloading it, I started the browser and was immediately confronted with a frozen tab bar. I closed the window and brought up a new one and I haven't been able to reproduce the issue. Maybe an Australis gremlin. Anyway, the speed is what I'm here for and to see for myself if there are any regressions. And I can report, in my non-benchmarking way, that the difference is monumentally, horrifyingly...nil. I honestly don't notice any speed difference between 17 and 24, and if anything, the GUI (menus, scrollbars, etc.) feels a bit snappier. So bullet dodged, and with IonMonkey more fully implemented in later releases, it looks like things will only get better.
The only difference I saw was in memory consumption. It was usually in the 300-350 MB range vs. 17's 250-ish range. I don't know if that'll be optimized in the future. But overall I'm happy and I'd say don't hesitate to upgrade when the final is released in December.
Also, a reminder for Leopard users, SeaMonkey-PPC and Leopard-Webkit are still making builds for PowerPC, so give those a shot, too.