I got hooked by FFVII and have been a diehard fan ever since, but the series has been having serious ups and downs.
I love VII, I used to hate VIII, IX is my favorite game in the series even after I've played pretty much all the older ones as well, X was so bad I completely changed my mind about VIII, and XII was about as enjoyable as VII except that it was the gameplay that was good and not the story. (I never got around to playing XI because I don't want to play an MMORPG no matter what it is.)
Final Fantasy XIII is the end of the road for me, though. If you want to know why, look at this:
http://www.vgcats.com/comics/
This pretty much sums up the general tone of every review I've read. By the look of things, they haven't CHANGED the gameplay so much this time around - they've just removed everything that ever made the series good.
The series has always been one of the biggest cash-cows out there, and because of this it will very likely carry on for decades to come. However, I'm giving it a rest for the time being!