I beat Final Fantasy XV (the main story, at least) tonight, and I can say that it's the first FF game I have actually finished. I have played every numbered entry in the series except for the MMO's, X-2 and Lightning Returns (if that counts) and until XV, I had never beaten one. I have been absolutely horrible at making an effort at finishing any of them. This is about how far I've made it through each one:
I: I'm on the final boss in the PSP/iOS remake
II: Barely played, made no significant progress
III: About 50% of the way through on a since deleted playthrough
IV: Favourite in the series, 80-90% through. Currently on the moon
V: 20-30% through
VI: 30-40% through
VII: 30-40% through
VIII: 5% through, also haven't touched much
IX: 20-30% through
X: 50%ish through
XII: Also barely touched, hated it, never going to try to finish
XIII: 80-90% through. On the 2nd last chapter
XIII-2: 20%ish through
As you can see, bad track record. Only reason I finished XV is because it was pretty easy. IV and X are the ones I most want to finish someday, as they're my 2 favourites. But once I stop playing an RPG, it's hard to go back to it. IV just requires a bunch of grinding where I'm at and I stopped X because I think I got a new game while I was playing it.
As for how I felt about XV, I thought everything before you leave the open world is absolutely fantastic. Had/having so much fun in there. But everything after you leave the open world was kinda bad, especially Chapter 13. Luckily for me, I only got excited for the game after the name change, so I'm not too disappointed after only waiting 2 or so years for the game. But if I had waited 10 years, I think I'd be disappointed in the game a little. Overall, might just barely crack my top 5 in the series.
