Wrong, the upgrade is $129. Go back to the store.
I use Jaguar, Panther and Tiger on a daily basis -- my desktop dual G4 has Jaguar 10.2.8 because of some incompatibilities with old software I still use, my file server runs Panther 10.3.9 and my new iBook G4 runs Tiger 10.4.2.
Apple will give you a long list of what's changed, but only some of it is noticible to the mere mortal user. I'll just note those below. I'll throw in Jag -> Panther, too.
Panther improvements over Jag (just the ones I notice or use regularly)
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New kernel, performance increases (noticibly faster)
Expose (window management)
Graphics subsystem performance improvement
Additional window classes -- more relevant to programmers
FontBook
Better Finder windows, including a customizable sidebar and volumes list
iChat AV
Pixlet video compression
Tiger improvements over Panther (again, a limited list)
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New kernel, performance increases (noticibly faster)
CoreImage GPU copper (although applications can't automagically take advantage of it)
Dictionary/thesaurus
Dashboard, although it's debatable if this is an improvement -- I think it's a waste of CPU time and I don't use any applets in it, but some people love it
More than merely token 64-bit support, but only relevant with a G5
QT 7 and H.264, although this can be added to 10.3 too
improved iChat AV
Spotlight instant search, and it really works! Wow!
Automator, an arguable improvement on AppleScript
Is it worth the money? I didn't think so, actually, unless you're really wowed by Spotlight (which I think really *is* cool) and Dashboard (which I think is a waste of time). On the other hand, I got Tiger with my iBook, and my iBook *feels* snappier than my dualie in some tasks even though the dualie wallops it in raw CPU bench. If you can find it marked down, it might be worth it. Wait a bit and see.
To those people running Jaguar, unless you have a good reason to stay on Jag (and unfortunately I do: some of my old Mac apps don't like Panther, and Apple progressively disabled old-style AFP volume support in Panther and Tiger so that my newer installations can't connect to my old Mac servers), the upgrade to Panther *is* worth the money. But I assume you're running Panther already.
"you're a doctor.... and 27 years.... so...doctor + 27 years = HATORI SOHMA" - RoyalWing, when I was 27
"Al hail the forum editting Shooby! His vibes are law!" - Osaka-chan
I could still be champ, but I'd feel bad taking it away from one of the younger guys. - George Foreman