I have been using a 15" PowerBook G4 (1.25 GHz, 512 MB, Radeon 9600) for the past 13 months, it's my first and only Mac.
I love the size and the build quality / fit and finish. The thing feels like it was machined out of a solid block of aluminum billet. Lots of nice features like the fold away lid latch and the backlit keyboard. Even the underside looks just as nice as the lid with only a simple battery hatch, ram hatch, and model details laser engraved into the aluminum finish. Mac OS X has been a fun experience, I like it a lot, but I also like Windows XP too. I originally bought the PowerBook for use on the road, for presentations, email, and web surfing. I started with MS Office 2004, but quickly started to use Apple's excellent Keynote application for presentations and Nisus Writer Express for word processing. Over time I started using the PowerBook for more and more tasks, these days I use it for almost all of my home videos (with iMovie and Final Cut Express) which I burn to snazzy DVDs with iDVD. I also sort most of my home digital pictures with iPhoto and sometimes do more advanced work with the Mac version of Photoshop CS.
Performance has been great, even with a "lowly" 1.25 GHz G4. Everything feels at least as fast as my old 2.2 and 2.4 GHz P4 systems, and usually much faster feeling that that. Because my digital photos are only 5 Mpixel in size, almost everything I do in Photoshop happens instantly, although some operations take about a second to complete. I sometimes play Sim City 4, Halo, WoW, and UT2004 on the PowerBook, they work great.
I'm still a PC user at heart, and I love building and upgrading my desktop rigs. That's also where I do most of my web surfing and email, because frankly a nice desktop monitor and full size keyboard and mouse is more comfortable than any notebook. I will probably always have a few PCs around, but I think I might also always have a Mac from now on as well. I'm loving Mac OS X and the PowerBook hardware.
There are rumors of a PowerBook update sometime this week. If you're waiting to plunk down some big money on an Apple notebook, I would suggest waiting until fall for the PowerBook G5. Otherwise anytime is a good time to spend $700 - $900 on a 1.2 GHz iBook depending on discounts and coupons.