I've logged a little over three hours into the main game thus far and I'm 26% complete.
Anyways, the single player is a bit disappointing, but not as much as I originally thought it was going to be. It's barely a Metroid game. It's an action game with adventuring tendencies, instead of the other way around as every other Metroid game is. There is some backtracking to open up new areas, as is finding weapon upgrades and some morph ball puzzles, but not nearly enough. This is the Metroid FPS we all were fearing when Metroid Prime was originally released.
Now if I put that bias aside for a second, the single player is enjoyable. It's a fun FPS, the best ever on a handheld.
But really, multiplayer is where this game shines and that is where the game will have it's longevity.