boring...
So people who've played it say it's an FPS. Ever thought about that they probably got to play a bit more action lvls and a bit less adventure gameplay??
It's only a name BTW, what's the big deal?
You could call it a FPS, but remember that FPS rely more on linear lvls and a lot of shooting and killing.
Metroid Prime has a inlinear gameplay and relies more on finding stuff to progress, which is typical for an adventure game.
So... although you could call it a FPS, a FPA would describe it better.