A fascinating concept, and great as part of a package, but nowhere near enough content to satisfy on its own.
You've also probably heard about the Weighted Companion Cube. A character that is literally a crate with a love heart on it, and appeared in the game for no more than 5 minutes, has become one of the most popular new characters of the year. The game also features one of the best endings since 2002's Mafia.
There is also a good chance you've heard about the games length, which prevents this game from getting an overwhelming recommendation from me. Some would say the length is fine, but if you were to buy this game individually over Steam, rather than as part of the Orange Box, you may end feeling a little ripped off. This is because the game's story is only around 2 hours long. Some would say this enough, that the game does not need to be any longer. I disagree.
First of all, the game hardly ever really challenges the player in the story mode, with most puzzles solved using one of a handful methods. The portal mechanic is not explored nearly enough in the story mode. Several puzzles hint at what is possible, but the game never really gets as intense as you might expect, and solving puzzles isn't as satisfying as it could be.
The story mode is complemented with two different challenge types, once the game has been finished. These are the challenge levels and the trial levels. The challenge levels are just levels from the regular game slightly changed to make the more difficult, while the trial levels are the same levels that are in the regular game, just with several parameters that must be met, such as finishing a puzzle with a certain number of steps or portals. These levels will actually challenge the player, but there is nowhere near enough of them, and it is a shame Valve did not do more with these modes.
The game also comes with a commentary mode, in which the development team discuss the creation of the game. It is interesting, and makes playing the game through a second time more worthwhile, but considered how polished Valve's games are, its surprisingly poorly implemented. The commentary is done by placing nodes through out the game which are floating speech bubbles that start minute long comments when the use key is pressed over them. Unfortunately, GlaDOS is much louder than the commentary nodes, and considering how much she talks, it is often difficult to hear what the developer is saying, forcing the player to stop listening to the node, and since there is no pause for the commentary, it forces you to re-listen to half of the node.
Portal is a fantastic experience, and complements the games in the Orange Box superbly. However, if one were to buy this game standalone, they might barely get a weekend's play out of it. There are plenty of games available for $US20 or less that are more compelling purchases. But, if price is not an issue, you still may be disappointed by this game's lack of depth, and although there is the promise of user made levels (for the PC) in the future, there just isn't enough here for anybody looking for a challenge.