A must-buy for everyone but Orange Box owners, but even they might want to consider it.
If you're unfamiliar with Portal, it's essentially a first-person puzzle game. If you've played Braid, it's kinda similar. What Braid did with time, Portal does with three-dimensional space. The goal of Portal is to get from the beginning of the level to the end. To accomplish this, you'll need to use portals, which can be placed on most walls with a handheld portal gun. Two portals can be placed in a level at any time, and anything that goes in one portal will come out of the other portal. You'll use these portals not only to get yourself from point A to point B, but also to solve puzzles. For example, there will be boxes you'll need to transport to buttons. When the button is held down by a box, a door will open. There are also energy pellets that, when guided through portals to their destination, will activate doors or moving platforms.
As the game starts, you'll wake up in a glass prison cell (or as a female computerized voice will call it, a "relaxation chamber"). A portal will open letting you out into the "test chamber," with its pristine white walls. Each of the 19 test chambers looks a lot like this, with the same white walls and rectangular shapes, but the visual simplicity helps keep the goals easy to find and makes visual cues easier to see. It also gives off this creepy "asylum" vibe, which fits well, since the computer voice, which talks to you throughout the game, is kinda creepy and bipolar herself.
While the puzzles are intriguing themselves, the computer, named GLaDOS, brings the game to life. The dialogue is well-written and humorous, from the lies ("This next test is impossible. Make no attempt to solve it.") to the promise of cake should you complete the tests. The gun turrets also have funny dialogue, and sound almost apologetic when you finally defeat them, saying things like "no hard feelings" or "I don't blame you." Oh, and GLaDOS sings a song over the end credits. While the puzzles are fun and the dialogue is entertaining, the main game is very short. New players can probably finish it in a couple of hours and people who have played it before can probably finish it in less than one.
Of course, if you already own The Orange Box, you already know all this. What does this game offer to you? Well, aside from the original game, there are 14 new test chambers, (which help offset the game's short length) although GLaDOS isn't in any of the new levels, so if you want more witty humor then you're out of luck. The new levels do include some new obstacles not present in the original game, but I didn't find them much of a challenge. The new levels should be pretty to figure out for anyone who's already played the original, but they are a ton of fun anyway. If the phrase "turret bowling" sounds like fun to you, then the new levels might be worth a look.
However, if you're looking for a true sequel to Portal, this isn't it. While the 14 new levels almost constitute a standalone game (considering the original only had 19 and the first 8 or 9 were just tutorials), without GLaDOS tying it together it just doesn't seem complete. If the new levels are enough for you, then it might be worth the $15, but if you enjoyed Portal more for the humor than the puzzles, you might want to hold onto your Orange Box. If you've never played Portal before, however, this is definitely something worth buying.