I installed on top of Panther and it just sat there on reboot. Used the "Upgrade" option. Re-installed a couple of times and same deal.
Final install I used the "create new system/archive old system" option.
Worked fine.
Have had no issues in 3 weeks.
Observations: USB file copies from things like thumbdrives and cameras is significantly faster.
Spotlight takes FOREVER to index, recommend doing upgrade late in the day so it can index overnight. Indexing on external drives is a bit cumbersome. Otherwise works very well. If you do a lot of graphics stuff you'll want to change default "command + spacebar" to F11 or F12 to invoke spotlight. Otherwise everytime you go for the magnify tool you'll get spotlight instead.
Dashboard is cute but you can live without it.
Menu bar has lost the Aqua look, looks like the menu bar from the "Milk" theme. I like it better.
Mail is improved.
Check out enabling Quartz, cool if your box is supported.
http://earthlingsoft.net/ssp/blog/2004/01/quartz_debug
http://arstechnica.com/reviews/os/macosx-10.4.ars/14
Running on a G4 1.5 mhz 17 inch with Hitachi 7200 RPM drive and 1 gb of RAM and the wifes 15 inch Ti-book.