Duff-Man says....Apple has said that 10.4 will ship "in the first half of 2005" so it is a ways off yet. In the past Apple has not offered any "upgrade" pricing except as part of the "up-to-date" program which only applies to people who have bought their hardware after the official shipping date has been announced. so, you can expect to pay the $129 for 10.4 next year if you want to upgrade...oh yeah!
Duff-Man says...I don't think we have seen the full feature set yet so it is a bit too early to tell. It sounds like for those with the newest hardware, the optimizations and new display code will make it worth it, at least to me anyway...you'll have to decide for yourself but usually the major updates are definitely worth it....oh yeah!