The PBG4 is warm! I picked up a CoolPad Podium Pad that I use underneath the computer. With it, the fan hardly ever runs, and it is a lot easier on my lap (rather than using me as an integral part of the cooling system).
The only other thing is that the machine tends to lose the little rubber feet off the bottom. That is probably because it runs warm, loosening the adhesive. Several people have used superglue to glue the feet on, and that seems to work great.
Neither of those two things are enough to sour me on the PBG4. I really like that if I want to run the CPU at full speed off the battery that I can, but if I let it run with the default battery options, then I can surf about 5 to 6 hours without power (or any other wire connected if I'm using the Airport card). It is more than fast enough to do all my development work as well as watch DVDs on. My previous desktop Mac (G4-450 dual-processor) was probably a little faster, but my PBG4-500 seems to keep up with my Athlon 900 on similar tasks. I also like the screen size (1152x768), which is great for development and DVDs. The screen also looks *good* at lower resolutions, unlike most of the PC laptops I've seen.
When I'm running Virtual PC for x86 emulation, I just don't expect it to emulate faster than 500 MHz, but it is easily useable for x86 stuff, as long as the program isn't terribly screen-update-intensive (ie. more recent games).
All said, I'm very happy with my PBG4 "TiBook". It works very well with both Mac OS 9 or Mac OS X (less than two-second wake-up time from sleep!). With Mac OS 9, I hardly have to restart, with Mac OS X, I rarely restart at all.