I never really got into the Gamecube Metroid games much, so I hesitated for a long time before deciding to try this one. It's turned out to be one of the best games I've played on any platform, I've played through is so many times, and still love going back to it.
For those not familiar with it, it's an excellent FPS with some good puzzle, platforming, and exploration elements, but not so much that it stands in the way of the action.
The combat sectiongs are probably the closest thing to the Halo series on the Wii, but the exceptionally well-implemented Wii controls make the controls feel so much better than any joypad-controlled FPS.
It plays a bit differently from most FPSs though, with more platforming, puzzling, exploring, and a smaller number of tougher enemies, making you work harder to defeat them.
The boss fights are exceptionally well-done, some of the most enjoyable boss-battles I've played in any game - but you do have to use strategy to defeat them, use your visor to scan the boss, and gain hints about weaknesses etc. - just using traditional FPS-style strafing and firing won't get you very far against most of the bosses.
The graphics are amazing - not just technically (although it is probably the best looking game on the Wii besides Super Mario Galaxy) but also the effort that has gone into the design of buildings, enemies, levels, textures, artwork - it is truly beautiful to look at in places (the planet Elysia in particular), and the music is very good in places (again, Elysia stands out).
It's less linear that most FPS's, you have a certain freedom to explore - there are gentle hints guiding you in the right direction, but on the 1st playthrough, you might sometimes find you're not 100% sure what to do next - it's the kind of game that from time to time makes you stop and think - but you can easily consult a walthrough, or look through the files you collect as you go through the game, giving you hints, as well as information and a quite-interesting background story of the long-extinct alien races who built the worlds you're exploring.