Being somebody who's both 1) stupid and 2) fearless, I upgraded to Jaguar today.

• My kensington mouse drivers produced kernel panics upon startup. unplugging the mouse fixed that.
• bash2.0.5(a), which I had built from off the gnu site, crashed on me when starting Terminal, luckily Jaguar has it's own version of bash, located in /bin/bash (in case you need to change things in NetInfo to get your Terminals to start again). this new bash doesn't understand dircolors.
• Mail still hates my Cyrus imap server. strange..
• Apple System Profiler now features crash logs. If that was there before, I must be blind. It's a nice touch.
• Rendezous is pretty neat. Stacy and I have been playing with it. iChat in general is pretty pleasant and much nicer than the aol aim client. (no more ads!)
• The orinoco wireless drivers for powerbook g3's no longer works, as many of you know. There are some rumors that they can be made to work. We'll keep you updated.
• The Dev tools for 10.2 come on a cd with Jaguar. Sofar, my old projects all build with it.
• gcc is now 3.1 which fixed a strange error in a compiler I've been workig on where a char* array returned from NSString (with lossy ASCII conversion turned on) no longer had \0 as the last element.
• Emacs (Carbon) 20.7 no longer works. Emacs (Cocoa) 20.7 doesn't work, either.

Basically, unless you enjoy misery, wait a few weeks to a month to make sure all of your existing OS X apps work with 10.2. I'm not unhappy I upgraded, though. It's very speedy.