A friend of mine said, "It's for die-hard fans only."
He's right. Luckily, there are roughly seventy trillion die-hard FF7 fans. And I doubt they will be disappointed.
Advent Children is a DVD-movie about the events that happen two years after the events of the game. Some of the story-related parts might get you a bit sleepy, especially if you weren't particularly fond of the whole Reunion/Jenova/planet thing from Final Fantasy VII, but that's okay because...
... the actions scenes are flat-out spectacular! Seriously, the fight scenes from the trailer were incredibly cool, but the ones that you see in the movie itself, wow. It's a good thing Square is really good at CGI, there's no way those scenes could have been done with live action.
Speaking of CGI, the quality of their work has definitely improved since Spirits Within. (Red XIII could use some work though.) I wish Spirits Within had had this level of awesomeness, but Advent Children has the advantage of a whole game of backstory with which the audience is already assumed to be familiar.
And for those skeptics who question how Cloud could possibly swing around that big-ass sword of his, better not watch this one as Cloud now fights with a big-ass sword in each hand!
If you are an FF7 fan of any sort, watch it. There are torrents all around, with subtitles.