It's great, for the most part. There ARE some areas which the game noticeably slows down, especially if there are a lot of characters and visual effects going on, but it's fairly rare, I've only see it happen in one or two areas (Like Macalania Temple, it noticeably slows down when you're in there).
It's pretty much exactly the same game, with slightly better graphics. Actually, the HD textures aren't always so good - Tidus's and Rikku's face can look messed up due to the hi-res textures at times. But seriously, it's worth your money if you liked FFX. I haven't played the X-2 remake. X-2 was where Square just lost me. I'm sure FFX-2 is beloved by many people, but it wasn't for me and every FF game they've released since then hasn't been for me either.