Just upgraded my 2003 GS430 with the new 2004 disc. Took about 2 minutes to pull off the plastic cover (velcro strip attached), and hit the eject button with the engine running.
I notice no difference in operation, although it did tell me the software was upgraded, and I couldn't revert to my 2003 disc. Some updated map information seems to be the biggest thing, but the lag time in road changes making to the DVD seems to be 18-30 months.