I was wondering if P3 is still worth playing for someone who has fiished P4.  Do the improvments made in P4 overshadow what is done in P3 so much that the whole experience feels lackluster?  Or is it still a great game to experience for someone who played and beat P4 first?

It probably won't feel lackluster. It's still a great game.
Set your Persona 4 characters to automatic and see what that's like. If you can handle that, you'll probably be fine with Persona 3. Oh, but be sure you play Persona 3 FES. Don't bother with the initial release unless you want it for collecting purposes.

Ohh ok, so one big change is the AI handles the other party members?  As long as they don't do anything crazy dumb I guess I would be cool with that. I would assume it would make it go faster too.  From what i've gathered though, it looks like P3 is even a longer game then P4.

Any other significant changes you can think of?

It won't feel lackluster I don't think..not to me anyway. I played P3 and P4 simultaneously pretty much before settling on focusing on one before the other and had no problem whatsoever. I find P4 definitely the better game because of plenty of improvements, but if you can get used to a few things different in 3 like no control over your party besides settings their tactics, and mainly having only one huge dungeon instead of multiple themed ones, I'm sure you'll enjoy P3 just fine. As Lord said, don't look for the original, just try to get FES.

I started with P3 FES because I figured their would be things in P4 that would improve and I wouldn't want to go back.  I don't know, but I don't have much trouble with the UI at all.  It just requires some memorization of spells and where items/skill are. 

The story is great though, so I think that is worth any inconveniences you may encounter.

It's probably not the best idea to get it now. The bosses and overall combat still hold up, but the Social links are nowhere near as entertaining, and Tartarus is much more reptetitive and frustrating.

I am enjoying it though the issues people have above along with the fact that you can't see what spells do outside of combat are annoying as they say. (the biggest one being no direct control over characters). The boss battles (most which actually happen outside of tarturus....) are all most more interesting really at least the first two, and the social links I have dealt with so far have been interesting. The girl from the shrine being a standout along with the treasurer from the student council..