Just returned from the Lexus dealer for a service visit. I asked about the DVD updates etc. They said that the Gen 3 version in MY 04 will look the same. A new DVD version will update the maps, but the overall system will be the same. The new GEN 4 version in MY 05 is a completely different system with a hi res screen, better color, more detail and so on.
So, if you update the DVD you'll get updated maps, but everything else will look exactly the same. That's no big deal to me anyway. I like the Nav and don't sweat about the small things - it does what I need, I just want to know I can keep the mapping up to date.
BTW - I spoke to a service mechanic about this system too and he told me to change the DVD is no big deal at all! Two screws to loosen the unit, press eject, insert new DVD, run it for (roughly) 1 hr to update and you're done! Now we just need somebody to buy the DVD and burn copies ;->