Terrible, terrible story and characters. Completely destroyed Samus as a character turned her from a badass silent killing machine to a whiny, insecure, frightened, submissive little girl straight out of some repressed Japanese business man's imagination of his perfect woman. Yes I'm aware of Fusion and the Manga but the way she is portrayed here is much worse.
But story has never been important to me in Metroid. The biggest let down was the linearity and bad design choices regarding power up usage and completely lack of interesting puzzles. Also the space station wasn't a very good environment for a whole game they really should have made the main setting an alien planet.
The good things were the speed of the gameplay. The combat and platforming are actually pretty good, very actiony and flashy. The graphics are pretty good especially through Dolphin but the federation soldier costume design is terrible, they look ridiculous.
Oddly enough I managed to find it somewhat enjoyable and finished it 100% but for a Metroid game it is a massive disappointment. If the game had of followed the Super Metroid mould but retained the same controls and combat (excluding first person) it could have been amazing. I really hope it hasn't killed the franchise as it's one of the all time greats.