So far life with Panther has been fantastic. The only downside was my I had to install two times as I was a little tired last evening and selected "Upgrade" rather than "Archive and Install", which I knew to do, but with little sleep and a crazy week I missed it. Doing this Safari did not load when clicked. I reinstalled this morning using "Archive and Install" and have a great machine. I also moved my "Previous Systems" folder to my external hard drive and got back 4.8GB. I now have 15.8GB of free hard drive (I get very nervous when it falls under 14 or 15GB and start fearing that I am running our of hard drive space).
So far the much faster interface and better responsiveness of Panther is a great addition. I am also loving the Exposé as I always have many windows open and this feature makes everything easier to find. The Font Book has been addition I have been anxiously awaiting and it has lived up to my expectations. The new finder took about 15 minutes to get used to, but I am very happy with the changes here and I think it has made the finder much more usable. I am also planning to use the file label colors to use this visual tagging to help me organize all the files I have generated and downloaded.
Mark Pilgrim offers a fantastic overview in his What's New in Mac OS X 10.3 Panther.