I recently sold my iBook/600/Combo to a coworker and have been considering replacing it with either an iBook 800 or a 12" Powerbook.
I really want to go with the Powerbook but I've got a few reservations. The 640MB memory limit is pretty sad. I don't understand why Apple would create a Powerbook with a base of 128MB of ram and then put another 128MB in the expansion slot standard. I imagine most people are just going to junk the 128 and pick up a 512 as soon as possible. The 12" Powerbook would be a lot more attractive with 256MB (or more) standard and an empty expansion slot.
When the IIvx came out it seemed like a decent machine but within four months it was replaced by the Centris, then a little while later by the Quadra, then eventually by the PowerMac 7100. The basic design proved very successful but the IIvx looked pretty bad compared to what you could get just a few months later.
Also, given Apple's current OS upgrade policy if I buy a Powerbook now I'll have to buy 10.3 later this year for full price but if I wait then it will come with the Powerbook.
For now I'm going to try and wait on the Powerbook until the next revision. Hopefully they'll fix the ram limitation with the next update and maybe update a few other features. (Or maybe I'll give up and just order a fully loaded 12" Powerbook next week.)