I replaced my 12" Powerbook with a MacBook for the size but then needed the Power of a MacBook Pro so purchased one as well. I like the portability of the MacBook (I have removed the optical drive and hard drive and it now boots from compact flash, as a result it is light and has a long battery) but it is a tad slower than the MacBook Pro. I also like the larger screen, backlit keyboard and Firewire 800. The MacBook is a 2.0 GHz Core 2 Duo with 2 GB RAM and the MacBook Pro is 2.4 GHz and it originally had 2 GB of RAM but has since been upgraded to 4. The graphics card in the MacBook Pro, especially under Leopard, takes a lot of load off the main process and RAM and therefore makes a big difference.
What I really want is a 12" PowerBook replacement as neither machine is perfect but for the time being the MBP see the most use with the MB is a bit of toy.
Nicholas