The interesting premise and outstandingly good voice work makes Portal worth playing.
Gameplay: The whole game is basically a huge puzzle, but not with it's test chambers. GLaDOS pretty much leads you through-out the entire game giving sometimes misleading advice as you figure out how to complete each test chamber. Though, you're probably not going to have trouble solving any of the puzzles the game throws at you, but they use the portal mechanic quite well using momentum redirection. The humor is no doubt the best part of the game, with every line being written and delivered in such unforgettable ways.
Graphics: Using the source engine, everything looks quite good. The models for the characters look nice while the textures are simple but clean. The style stays fresh through-out, and the game scales quite well with older computers.
Sound: Definitely the best part of the game. From the voice acting to the sound effects - you'll really enjoy what this game has to offer in the audio department.
Value: For $20, it might be worthwhile for about a weekend worth of gameplay to get the most out of this game (Time trails, achievements, etc), but if you're planning on getting the Orange Box, it suits the package perfectly.
Tilt: Overall, Portal is a good game, but it doesn't offer much of a challenge. The interesting premise and outstandingly good voice work makes this worth playing.