I'm a big fan of Final Fantasy X-2. It gets a lot of hate, unfortunately, and I have my own theories as to why that is, but I found it a very enjoyable game. It obviously wasn't deep and sad like the first one,
I dunno, I think the Shuyin/Lenne plot was pretty deep, especially as its portrayed in 1000 Words.
But I agree, I really enjoyed X-2. A lot of it is due in part to the fact that, as Phazon said, it's campy and and has a lot of fan service. The paradigm shift from X to X-2 was just too staggering for people and doesn't really reach the same audiences. At the same time, its concept is not only fun, but it also puts the length of the game more in the player's hands, not only giving them a lot to do but also allowing them to choose how long they want to do and how much they want to do before they decide to end the adventure. Not too mention that it puts the spotlight on Yuna, who the spotlight was on in FFX despite the narrator insisting that it wasn't her story, which would probably have been more of a pro if the game was more serious.
Actually, I like the fact that you mentioned Gundam Build Fighters, because that is the
perfect example for a comparison to this game. FFX-2's tone is so different, and even the way it goes about story progression and exploration has taken a massive shift, from the semi-linear traditional point-to-point exploration to an almost sandboxy, quest filled world filled with characters from any side of the spectrum. Even the gameplay has taken a shift towards the more dynamic dressphere system that is dramatically different than anything seen in the series before and...well, any concept introduced, by both being ridiculous in concept and at the same time attractive (conceptually, not aesthetically) and creative.
Just like Build Fighters and G Gundam before it, Final Fantasy X-2 is experimental in every sense, and I think that's what makes it fun- to play and to experience. It's something new and unexpected from the company, and you just don't expect something so off-the-rails.