I've been running Panther for a few days now and don't have too much to complain about. The upgrade/install was flawless (and rather quick). Truth be told, I would have paid $129 for this 'upgrade,' but I'm glad I only had to pay $20.
The feature that I am most impressed with and one that I've talked about quite a bit in the past, is Exposé. It's just one of those features that makes you wonder how you ever lived without it. I've yet to "feel" it get bogged down and I've thrown upwards of 35 windows at it. Apple also (finally) 'fixed' command-tab application switching. Anyone familiar with Windows and most modern *nix window managers, knows that command-tab allows you to quickly cycle through your open applications, a very useful feature and one that was sorely missed when I moved to OS X. The pre-10.3 implementation cycled through your dock and was useless if you ask me. Jason Kottke proposed a great idea concerning the merger of command-tab with Exposé.
Mail.app has gone through a significant overhaul as well. It has a much faster feel and a slightly more polished look to it. The big change is the addition of threaded messages, which is something it should have supported from day one. A big change for me has been the "Place signature above quoted text" option. You see, before, when I replied to a message, I had to manually cut/paste my signature each time from below the quoted text to above it where my message went. I realize that Apple's implementation comported with the RFC, which requires that message replies go on the bottom and that signatures are the very last thing in the message (this has been carried over from Usenet), but I think it is rare that anyone complies with that anymore and I'm glad to see that Apple now lets me abandon it without too much fuss.
The Finder has also received some significant enhancements, including a major aesthetic change — it now has the brushed-metal look. I've heard a lot of complaining about this switch, but I love it. In fact, I wouldn't mind it if the entire interface moved to brushed-metal .
On a slightly related note (by way of the fact that I'm running Panther on my new PowerBook with built-in Bluetooth), I got a Bluetooth mouse last week and couldn't be happier with it. So nice to be able to pull my notebook and mouse out in class and not be encumbered with wires.