Update: I was unfortunately not aware of Shamus Young's severe criticism of Fallout 3 available here to link in the original piece and I regret that. It dovetails rather nicely with what I've written and it's much better executed than my piece. I strongly recommend anyone...
The Good: Superb visuals and audio, links with Metroid Fusion, lots of exploring, fun boss fights
The Bad: Doesn't evolve the formula, constant backtracking is tedious, lousy save system, funky controls, no multiplayer, getting 100% is nigh impossible without a FAQ, not enough Morph Ball sections
I could never really get into the Metroid series due to it's lack of a decent story. While Samus Aran is a gaming pinnacle there's not much behind her besides she's an orphan raised by the Chozo race of Tallon IV. A "Great Poison" came to the planet from a meteor impact and ever since the Chozo had to flee their homeworld. Samus, being armed with Chozo armor and weapons, attempts to save the world from the Space Pirates and jellyfish...I mean Metroid.
The game consists of five huge levels that you go back and forth from finding upgrades to your suit. There are a total of 4 weapons, 4 weapons upgrades, plus a few Morph Ball upgrades etc. My favorite part of the game are the cool Morph Ball sections yet there aren't enough of them. You can, eventually, use the Spider Track Ball upgrade to click to ceilings and walls and there are some really cool segments for this yet only a few. You explore each level by needing to use a certain upgrade to get through such as a door needing a certain weapon to open it. You can also get missile expansion upgrades, power bomb upgrades, and energy tank (life) upgrades, but most of these are extremely hard to find and only die hard Metroid fans will spend the time finding them. On the other hand there are tons of different enemy types and cool bosses. The game is just about exploring and now that it's in 3D Nintendo really makes you explore.
The controls are "dumbified" for Nintendorks who were used to the 2D era and didn't know how to control an FPS so there is a lock on system (that is a bit annoying) and you use the L trigger to move your reticle. This can be really annoying for people used to standard FPS controls. You have to always stop to find out where to jump while enemies are attacking you. First hint: Nintendo this doesn't work! Secondly, there are a lot of different types of environments from water, to ice, to viney green lands. Once you find your Gravita suit you can run through water with no problem so in turn you can explore more areas. After you get all of your add ons you have to find twelve artifacts to get to Metroid Prime.
Basically, if you've played other Metroid games this is exactly like those except in 3D. I hated the constant backtracking and lousy save system. You will be in one level but the next add on is two levels away. Instead of warping there you have to run through every single level and the enemy respawning doesn't help either. The best thing I liked about this game was the visuals. They look amazing for a launch title, and what's amazing are all the effects. Water and steam will hinder your vision, there will be static on screen when your electronics are disrupted, Samus' face will reflect off her visor. The sound, on the other hand, isn't so great with Ecco the Dolphin esque music and creepy techno. While some of it works it just sounds really old.
There is also no multiplayer at all so this is another disappointment. You can link your Metroid Fusion GBA game for extra stuff, but I couldn't try that out. The game also sports the lamest endings ever (and I got the second best one). Overall, if you hated Metroid before you still will, and if you loved it before you'll love it more. Newcomers stay away if you don't like your FPS formula messed with. I don't hate Metroid I just wish it would evolve more.