Oh man, I loved XII so much. I need to replay it sometime soon. Mostly, I appreciated the fact that the biggest part of the plot was, just this once, not about romance. I love a good political story (hence my love for Suikoden) and I was nuts about this. Most of the negative comments I've heard were about the battle system, but I liked that as well. I enjoyed that they finally stepped away from the little loading bars and tried something different (of course they went right back to the ATB bars for XIII).
As for XIII, I haven't played it yet due to not having my own consoles and it being a little too awkward having to mooch off my brother. But I'm also not very motivated about it. I bought XII a day before official release and my girlfriend promptly lost me for the rest of the night. Now... I'm getting such mixed reviews that I'm not in any kind of hurry. Maybe if I ever get my own 360 I'll pick it up secondhand.
What bothers me about it, though, are the Versus XIII and Agito XIII games. They aren't sequels or prequels, but set in the same universe, but Versus would (for now) only be on PS3 and Agito would be for PSP (which I also don't have, DS forever) but only released in Japan. We're taunted with extras, but they always cost too much, in my eyes.
As for XIV, we'll have to see. I don't usually pay attention this early on because it'll be forever until release and you can't really tell anything from screenshots other than "oooh, pretty".