The speed gains are much larger on Intel machines, but the PowerPCs will also see a speed increase, as normal with newer versions of OS X.
I've been using a beta version of an external HDD on my MBP, and it's very good. I have two gripes though:
They've replaced the arrows under dock icons with horrible little glowing lights, which I just don't like at all. They look really naff IMO. The Dock separator, which is just a gap in Tiger and now resembles a zebra crossing also looks silly.
The new close, zoom and minimize buttons look odd IMO. I think they've slightly overdone them.
Other than that, I can't wait for Leopard. I'll wait till .1 or .2 before I switch. I need Logic and Reason to be incredibly reliable before I make the move.