I think Elchebib is right..FFXIII was only a great game if your new to the series, amazing graphics and gameplay but doesn't feel like the old FF series, characters aint as memorable as FF7-8-9 and 10, and sure thing story aint as deep, besides who needs to play a linear FF game..i guess FF was great with its open world environment.
Funny. I used to say that FF8-9 were really bad compared to the "real" Final Fantasy like 5 or 6. I guess it is normal evolution. Each Final Fantasy is a new thing. If you expect every game to be exactly like FF7 (or FF10) you will be disappointed.
I haven't played XIII yet, so I don't really know if it is bad, but looking back I think I loved every single FF game I played:
FF2: Most intelligent leveling system ever created.
FF5: Nice little game, you fall in love with the characters.
FF3/6: First time ever played such an epic game.
FFVII: Amazing graphics, epic story. Cloud/Sephiroth is still my favorite hero/villain couple of all time. Figthing system and level up was standard though. Killing Ruby weapon was no easy.
FFX: Most emotional FF. Story is amazing (unlike 7 it's not complicated to understand). Graphics are mind blowing. You
will fall in love with Yuna. You
will want to play with Lulu's boobies :) Fighting system is stop time (for the first time) which implies some strategy. Leveling up with the Sphere Grid is interesting system, although it could be improved. Aeons (summons) are simply glorious.
FFX-2: Nice little game. Story is cool, girsl are sexy, fighting system is fun (dress spheres). Ultimates are fun (super dress spheres). Main story is short and concise. A
lot of side quests.
FFXII: Poor story line. Very short. An infinite number of side quests. Most intelligent and enjoyable fighting system of all time. With Gambits, you basically have to create your own AI. 5h long ultimate boss fight.
I did not mention 8 or 9 because I only spent a few hours playing each and did not really enjoy them back then (for the same reasons you don't like XIII, I used to say the
same things). I really want to play them now.