Well, I've been waiting for the game for quite a while and it's really worth the time.
As for the FES version of Persona 3 - there are (I think, seemed so to me) new cutscenes and a few new social links, outfits for the characters, new stores opened, weapon fusion and new personas + you can keep all your items from Persona 3 (Though on the key items.. I didn't have any of my weapons or any of my maxed out cash left :s ), oh well.. The difficulty is just right, tho I think I had to stay a little longer to get my lvl but I don't think it's an obstacle for a good game :)
If you haven't played Persona 3 - don't bother, buy P3 Fes instead, you gonna get an enchanced version of Persona 3 + a new storyline :p
I chose the 'Hard' on the enchanced version of persona 3 and still it's proven to be hard.. Played P3 for 180hrs and I'm not going to back out on P3 Fes ;)
This game should be in anyone's collection ;)