I upgraded 2 TiPowerbooks so far, and both went fine.
That being said, if you have been experiencing any strangeness or instability in your current install, probably an archive and install would be the option to pick. (this saves your home folder and apps along with most application prefs and settings, but installs a clean version of the system to work with)
If you use any haxies or low level system mods you probably want to disable them before installing the new system version.