I bought FFVII and seriously could not stop playing for months. 2 and a half, I think.. the "final" game I made ended up with something like 175 hours (or more.. I forgot) with all the chocobo breeding I was doing.
At first I was reluctant to buy FFVIII because of the lack of the materia system that I thought was so cool.. I ended up buying it though at first only for the stunning visuals (and the fact that it was an FF title).. which was strange because I prided myself on not buying stuff for just visuals. I'm glad I did, though.. 'cause VIII has to be my favorite game.
I couldn't say a common factor that makes me love FF games in general, though.. one reason being that I thought IX sucked.. but mainly because they are very different games.. besides being RPGs, having similar play aspects (worldmap, stuff like that) and all having chocobos, there isn't really all that much common about the games.