I use it on a Mac Dual 1.8 and a Dual 2.0. No real issues for me.
Going from Tiger to Leopard was a slight performance hit but not much.
I'm using HD3.
I usually run 2 instances of Waves IR-1 and an instance of SSL compressor native. Everything else is TDM.
I never use more than 5 or 6 virtual instruments either.
Locked to Reason works well too.
Take the plunge but buy a new system drive.
Then go directly to the thread that complains about all the dark grey.
