Ever since the Gamecube (even on the DS) Metroid games have been largely First-Person. A very cool departure from the original side scrolling, platformers that preceded it. The team who put out all of the "Prime" games did a really good job of marrying the adventure/platform, find-your-scattered-gear-to-gain-access-to-previously-attainable-areas style game that Metroid is basically famous for. Prime, Prime 2 and Prime 3 were the perfect blend of Super Metroid style exploration and Halo style heads up combat. Other M takes it back to the side-scroll platform days, which was something I didn't at all expect.
The game takes place in-between the SNES classic, Super Metroid and anything that happened during the PRIME era. It was designed by Team NINJA, the guys who revamped the Ninja Gaiden series for the XBox. It feels a lot like a Ninja Gaiden game, as far as how the maps are laid out, how the enemies come at you, and how you dispatch them. The best part about this is it also holds true to the extreme exploration, giant-boss-battle, find-the-weapon-to-open-this-door type of gameplay that I just never seem to get enough of.
I haven't finished it yet, because I acquired Skyrim for xmas and haven't played anything else since. But I am probably about half way through it and I can say, with confidence that if you're a fan of any of the previous games and want a solid change of pace, or you just need a fun game for your Wii that isn't Mario, Sonic or Zelda, pick this up.
I kind of wish Team Ninja would pick up Castlevania and give it the same treatment.