What I have found so far, as a newbie Mac user, that G4 processors are surprisingly snappy. I found my new 350MHz G4 Sawtooth with 256MB of RAM is much more snappier and quicker than my 1GHz Pentium III with 382MHz of RAM running Windows 2000.
Now comparing Apples with Apples, the biggest visible improvement that I saw was from the Quartz Extreme enabled videocard. My G4 Sawtooth and G4 cube are similarly equipped, but the cube has Radeon videocard vs the original Rage 128 from Sawtooth. The difference is quite amazing. Panther is much more enjoyable with Quartz Extreme enabled.
That said, I just took the delivery of my G4 Powerbook with 1.2GHz processor today, and it's funny that I don't feel that the Powerbook is that much quicker than my cube. But then, my use of these computers are 90% internet surfing and browing. If I try video processing or program development on these machine I could probably feel the difference easily.
A side note: If you are getting 15" Powerbook with Combo drive, be sure to order illuminated keyboard. It's a $60 option that will turn your everyday use of the laptop experience priceless. I think this should be a standard equipment!