I am re-installing everything. My Powerbook's drive died, so I had to start from scratch (and the partial backup I could recover). I've switched from Tiger to Leopard, but still have iLife '06 for PPC on that machine.
On the iMac, I have the more recent iLife '08 which is nice (especially iPhoto, which we used to reorganize most of our family snapshots). Garagaband seems to be improved somewhat, but not dramatically. I have not had the chance to try others.
I'm still mainly using Ableton Live 6, but had a hard time getting the m-Audio Firewire drivers installed on the iMac (new drivers, new firmware, several reboot). It did work in the end. Certainly not plug and play.
But now I have to decide on the use of software and plugins. I am hesitating to reinstall all VST and AU plugins (effects and instruments) just because I have them. I'd rather stick to a minimal set of well-behaving default components. I'm even considering using only what comes with Live (+ Operator) for compatibility: Powerbook PPC, iMac Intel and maybe XP or Vista too. Maybe less is more. Even when using only Operator, there is such a wide range of sounds I can make. And I'm a guitar player anyway.