The features I switched to 10.3 for were the background iDisk synching, the improved finder windows, Fontbook, and Expose. I'm still afraid of Filevault, but I also don't have any real reason to use it. There have been some real improvements to Mail (don't know whether these are available to10.2.8) and everything is definitely *much* faster. For me, it hasn't been any more or less stable. It did "break" at least one application (a non-critical one, MS Links, which now only runs in classic mode). Also, Fontbook rearranged all my fonts, which was definitely not a good thing.
I guess it was worth it for me, but probably only because I got it for $69 with student discount. It would be interesting if I could "magically" switch back to 10.2.8 and see if there are other differences that I'm just not thinking about. Everything works great in 10.3, but most everything worked pretty well in 10.2.8.