This is a very old review. I wrote it back when the game came out, and then modified it a bit a couple of years back. And now I am posting it for your reading enjoyment. (Also because Anandxxx asked me to since he loves the game so much.) Enjoy!
Awhile back, Nintendo made the announcement that we had been waiting years for; a Metroid sequel was finally coming to their home console, the first since Super Metroid on the Snes. Suffice to say, we were all psyched. But then came another announcement, one that filled us with fear. The new Metroid would be in first person shooter form, and handled by a start-up developer, Retro Studios. It was almost certain to be a letdown, if not totally suck. Yet we were proven wrong. Retro decided to blow away all expectations and make a faithful, polished, and utterly amazing Metroid game, which converted even the most diehard dissenters.

If you're looking for a traditional 1st person shooting game, let me warn you right now: Metroid Prime is not that game. The dual analog "free look" found in most FPS games is gone, replaced with a more platform-centric control scheme. The game focuses as much on platforming and exploration as shooting, so the controls work well.
You start off exploring a fairly linear space station that may be a bit reminiscent of a typical FPS, but after that stage is finished, you are thrown into the middle of a planet, and the exploration truly begins. The design of the planet is one of the most amazing things I have seen. The game has five main areas, which are huge and complex, and cannot simply be made through, but have multiple branching paths, hidden areas, secrets that must be passed by and come back to later with new power-ups, etc., and all are interconnected to each other, often in unexpected ways. Even from the first landing on the planet you will see countless areas you cannot yet get to; a ledge just a bit too high to jump to, a door you cannot open yet, a small hole beneath a jutting rock. The joy of finally accessing the areas you have been teased with through large portions of the game is unbeatable. And us usual, there are various missile, bomb, and energy tank expansions hidden throughout the world.
Metroid fans know that a good portion of the coolness of the franchise comes from the various kick-ass power-ups, and in this respect Prime does not disappoint. There are beam weapon, visor, suit, missile, morph ball and other various upgrades. Perhaps the coolest new upgrade is the "spider ball", which lets Samus attach to various spider tracks in morph ball form and roll up them to previously unreachable areas.
The combat is handled with a lock-on function much like the Ocarina of Time. Enemies generally cannot be simply locked-on and blown away though; many have specific areas that need to be targeted, or other tricks to defeating them. Generally scanning an enemy with your scan visor will give some hints, but often you will just have to try things out until you figure out what works. Boss battles are especially impressive, huge and epic (and often very difficult!) battles with various techniques required, they are definitely some of the most memorable battles out there.
Metroid Prime is by far the most amazing graphical feat on the Gamecube. Every single room is packed with detail: huge knotted trees with intertwining roots and moss, crumbling ruins with sand pouring out holes, ice covered lands with crystals hanging about, enemy bases with huge complex technology everywhere, it is absolutely beautiful. And the game blends the natural and technological styles together for one perfectly coherent and inventive style. Most importantly, the attention to detail leaves every room truly having its own feel, so you never feel lost in mazes of repetitive design. The musical score of Metroid Prime is excellent as well. Lush, beautiful, and incredibly atmospheric music plays throughout the game; bold at times, discrete when it needs to be, it comes together as one of the best overall sound experiences on the Gamecube.
Simply put, Metroid Prime is the best Gamecube game to date, and one of the best video games ever made. It's really that good. A perfect blend of classic Metroid gameplay with all the new elements a vast 3D world and first person perspective bring along with it. Don't expect a first person shooter, this is so much more. Don't expect anything, actually, because I can guarantee you that your expectations will be off. It simply has to be experienced to be understood.