From Wikipedia: Portal* is is a single-player first-person action/puzzle video game developed by Valve that is often boasted by fans as being more popular than the main fare of the Orange Box (Half Life 2). It was released as part of "The Orange Box" for the XBox 360 and Windows but will be released on its own this April 2008. It is a short game consisting of 19 levels, taking anywhere from 3 to 5 hours for first-time players to finish (I took about 6, spending time exploring and experimenting with the portal gun).
*Note: be aware that the Wikipedia article is full of un-noted spoilers. Read at your own risk.
Setting: A sterile, white & grey walled "Enrichment Center" for "Aperture Science Laboratories".
Premise: You are a woman named Chell. You wake up in the Enrichment Center and are prompted by a female robotic voice (GLaDOS) to begin your training process by completing a series of puzzle tests, one per level. Generally, the test is to cross a room, overcoming various barriers, and/or manipulate a few buttons in order to gain access to the exit door. The puzzles are often very simple but require lateral thinking to discover the answer.
Early on, you obtain the "Aperture Science Handheld Portal Device" (portal gun) which, as you guessed, creates portals. Portals come in two colors: blue and orange - the color difference is there only to aid the player in telling the portals apart; they otherwise do the same thing. Go in one, come out the other. Once placed, a portal stays put until the player shoots to move it elsewhere. Only one portal of either color can exist at any given time.
I played this on an XBox, using a traditional XBox controller. Both control sticks are required for movement - one for forward and side-to-side movement and the other to aim the portal gun. It takes some getting used to (and a large screen helps) but with a little patience it becomes second nature. A friend of mine watching had previously played Portal on a PC with a mouse and decided that was the way to go - particularly for aiming.
I am a long-time MYST player and the more mechanical levels of Portal will be right up any MYST fan's alley. But the majority of Portal revolves around using the portals and physics to get across many of the more challenging levels, particularly with the "speedy thing goes in, speedy thing comes out" mantra. Because you cannot run or jump, you have to harness physics (usually the physics of gravity - i.e. falling) to create speed to accomplish some amazing feats. Because the game is in first-person perspective, these feats become truly stunning and kept me on the edge of my seat.
Also, for those of you who hate dying in games, its pretty hard to die in this game through most of it. Luckily you cannot fall to your death - you can only drown in sludgey water (where applicable), be crushed under heavy objects (again, not a common situation, or take too much sustained energy damage from lazers in two or three levels, but again - avoidable. You pretty much never have an opponent other than the puzzle itself.
Storyline: because this is ultimately a mini game, there is only a basic storyline. What really drives the game forward is the mystery of the facility, GLaDOS' motivations, and the amount of humor in the game. It uses lots of techno jargon in silly ways, is full of things that later become inside-jokes to Portal players - and heck, the supposed reward at the end of the game is cake. What is there not to like about that? :D
I almost wish I had my own XBox just so I can play Portal. Maybe on my next Mac I will break down and install Windows via Parallels *just* so I can play Portal. The creators at Valve intend on creating a full blown Portal game of some kind and I will be on the wagon to get it when it comes out.
I honestly can't find anything bad to say about Portal - even about its short length, simply because they used every level to its fullest (particularly the last two) to where any more would of felt tacked on. The visuals were clean and were never glitchy, the sounds were practical, the story was funny yet mysterious, and the controls were smooth and pretty easy to use. Portal is a very unique game can appeal to a varied audience and is worth at least a rental. Go try!
And for those of you who *have* played, here's two shirts I made: