I work on a G4/733.
No problem issues here with SFP.
Just Cubase and ASIO sometimes cause a simple crash, but never SFP.
Besides ASIO there's OMS (Open Midi System, Opcode) which is a bit tricky to implement, it has it's own type of interface, not really mac like.
But once you got it working right, it will be fine.
The price you pay to be sure about your hardware (in it's combination also!) is: higher basic prices (though you can doubt that if you read the problem forum and see what people have to invest in their pc's sometimes...), high Apple repair costs in case of, and most important: an unsure future about CW and mac.
There is the question of wether CW will ever make SFP compatible with the superb OS-X, and second, if CW will make the cards compatible with the new PCI slots of the G5, which have a lower voltage than the cards do.
Also you have to accept that the older macs (all of them not being G5) can't compete with pc's of the same time generation concerning the number of MHz, but at the same time you may double it to get the pc equivalent
BUT: you don't have to face the main pc issues of 1. what mobo you should take, 2. the ACPI layer issue, 3. the IRQ issue and 4. HT (Hiper Threading) issue.
Even the very skilled pc users here sometimes have to face desperate making mystery problems, not even knowing if it's the hardware or the software which's causing the trouble.
Mac isn't the port to heaven, but I don't know a better machine for making music, and concerning the OS, it is very accessable and understandable.
In case you like to change or add hardware parts: you can do everything you might want, except change the MoBo, thank God
Join the stupids, Creamware loves the mac
