It has a number of flaws, including the entirely predictable plot, heavy-handed moralising and bum-aching length.
However, it really is as spectacular as it has been hyped. It's easily the most visually impressive film made yet. It's well worth seeing on a big screen just for that, and if you do, definitely worth going for a 3D showing.
Given the setting and plot though, I couldn't shake the feeling I was watching what a Final Fantasy cut-scene would look like if Square-Enix spent an order of magnitude more money on them.