I perfected the Sphere Grid and beat all the Dark Aeons and Penance... and then my friend saved over my 300 hour game by accident.
I thought the sphere grid was a good system. It allowed for some sort of flexibility later, while still making all the characters somewhat different to begin with. And it makes the game entierly stat-based, which means you don't have to spend time buying new weapons from every shop every time... not that that's a bad thing in itself, but it was something different and quite fun to play around with.