I've been using a 17" Powerbook for almost 4 years now. I can honestly say it has, without a doubt, replaced any need I have for a desktop machine over the last 3 years. I am able to take all my work with me to client meetings. I have my laptop rigged up as complete development server for testing - see Dave Shea's excellent article on virtual domains within apache on a Mac. We can demo any of our applications in a controlled environment for clients and prospective partners. So now, whenever I am on the road, short of an internet connection, I'm actually still in the office.
Granted we still have an office full of desktop PCs for various applications, testing platforms and at home we have a PC for web/email/work for my wife, but given the line of work we are in, a laptop makes just makes more sense.
When I first got it, people would stare in awe as a would open it up a meetings to take notes on. They would ask me what I was doing, and I would say, taking notes. Everyone would then look at their boring old pen and paper and back to my laptop with envy.
This is now the first time I am actually thinking of upgrading my trusty 17" G4 Powerbook for real. Behold the new 17" MacBookPro. Commence drooling.