I watched the Spirits Within before and after Advent Children and felt pretty much the same about it. I didn't feel the characters were real at all, I felt like I had missed their entire backstory and the entire main plot. It was like I skipped to the end of a story. But that's necessary when you're doing an hour a half movie with no source material of back information, so I get over that.
But even then, the plot and everything about it is just kind of average to me. For every neat idea squeezed in there's a lot of times I was just bored. It's not a bad movie and it isn't the anti-christ or something, but as far as sci-fi movies go it was nothing special. The villains have arched brows and do what they do for relatively no reason, there's a love story injected into it but barely covered, there's a cast of funny characters that all die, and there's bad alien creatures menacing everybody.
So basically it was crappy Aliens.
But again, it isn't a bad movie at all, a lot of things about it are good. If you do hate it, try subtracting the FF from the title and only looking at it like a sci-fi movie...There is enjoyment to be had sprinkled all over it.
And let us all be amazed that nobody's hair moves but Aki's... because she's a witch