Looks like I'll be the first one in this thread to say something like:
Panther sucks. It gives me modem errors when trying to connect on dial-up sometimes (extremely annoying), the Finder windows have all my icons spread out way too far apart as opposed to Jaguar, the Finder windows are way too large especially with the Sidebar open, the Finder window toolbar customability has been reduced to "hold the icon over the top of the window for a second and only then can you add it...just without the name of the file/folder," and the Search toolbar in Finder windows doesn't work if you have to click the "chevron" ">>" symbol to access it. That's just from the Finder and menu bar, hence, the Finder is an aggrivating downgrade in usability.
Sometimes fully downloaded images won't open in Preview, Safari for Panther has always had more stability issues than 1.0 of Safari for Jaguar, simple machine to machine networks with a single reverse ethernet cable is much harder to set up with Panther, and trying to work in the Finder with column view is horrible, because the columns keep jumping back and forth. Additionally, sometimes if you want to open a file in a certain app. by using the "Open With" menu option, the menu to the choice of apps never shows up until you move the file around on the screen and try again. Also annoying. I never see how all the review sites give 10.3 the best OS of the year award, when it has so many more [annoying] problems than Jaguar.
These are the only reasons I see upgrading to Panther:
[1] You need to make PDFs for free
[2] Appleworks can't read .doc files
[3] You find yourself going to the desktop a lot for files (which you shouldn't be if you stay organized)
[4] You need to play music through your stereo from iTunes and AirTunes
By the way, the Chess program in Jaguar is funny as all hell, because it's so funny to play with its instability. Panther's version of Chess is stable, so no more comedy with Chess in Panther. (You should be using Sigma Chess 6 for chess, not Chess.)
Also, some people might say that Panther is faster than Jaguar. Well, [1] in terms of boot time, it's only no more than 10% faster, [2] Finder navigation and starting up applications are maybe a slight bit faster, but...with the many little/medium problems in Panther, overall, you'll probably waste more of your time than you would with Jaguar.
So if none of the above mentions get you motivated to upgrade to Panther, or if what I said is a turn off from Panther, then don't waste your cash, because it is not worth buying.
Definitely wait for Tiger to come out before spending the $129 (or the equivalent for other services).