Truly magnificent game. Shame there's hardly any of it.
First, the good. Portal is not just funny, but occasionally hilarious. Whereas the ordinary game might make you smile a couple of times, Portal is a riot. Well written and full of funny dialogue, its surprisingly light-hearted tone makes it a joy to play. The puzzles themselves are challenging, but you won't feel like you're in over your head. There are a few stonkers in there, but most feel just right to test you. The game is essentially a FPS without fighting, but you'll still encounter a few things that don't seem to appreciate you. These too, must be dealt with, and are blended well into the puzzles. One of the big positives is what must undoubtedly be the greatest end-game song ever.
Sadly however, Portal is not perfect. One glaring omission seems to be most of the game. Yes, Portal is so short it feels like the first chapter of a longer story. About half (at best) the length you might expect, it will only last a few hours, and because of the nature of it being a puzzle game, it doesn't have a whole lot of replayability. I could ramble for hours about Portal, and dull as it may be, it wouldn't matter, because you'd still have time to beat Portal.
In short (just like the game), Portal is a game you should definitely look into getting. However, the fact it is so brief means it ultimately feels like a missed opportunity. You may prefer to pick this up as part of The Orange Box, to give you something to do that will last. I can comfortably recommend it, either as a solo game or as part of the excellent The Orange Box.