Panther!
I just finished installing Panther. I'm still trying to pry my jaw off the floor. Wow!
I can't imagine the install being any easier. I took the "Archive And Install" route with the "Preserve Users" option which basically moves the old system files to a temp directory and then installs Panther anew, putting my home directory back in place. It just worked! All told, I think it was about 30 minutes of downtime as it installed a country mile of bits, and during that time it required about 1 minute of my attention.
Now to be fair, since I bought this PowerBook recently, I qualified for the $19.95 Panther upgrade. Would I pay full price for Panther? Well, you know when you go to buy a mattress the salesperson always tries to hook you with that line about spending half your life in bed, so you want the best mattress money can buy? Pay no attention to that nonsense. After all, you're asleep! Unless your current mattress is keeping you up at night or putting a crick in your back, you probably won't notice a difference in the mattress they want you to buy.
But I spend most of my waking life wandering around this operating system for business and pleasure. It's a sad state of affairs, I realize, but the point is the new features will pay for themselves many times over in my case. Indeed, Expose and the revamped Finder are already helping me sleep better.