It's a great game, but like negative reviews say, it's short. But it's part of the story, so I think this shouldn't be left out.
Pros: - Story, likely like intro for Portal 2 - The idea; unique, this game brought so much, without this we wouldn't have Portal 2 and therefore not fanart, fan vids, fan mods to other games etc. - It's one of the greatest puzzle games, little bit easy but it's good - Speedrunning, see the speedrun vids, this game is pretty decent for it. - Color theme is beautiful, there's so few games with only white, brings the unique exprience this way as well.
Cons: - Short
Conclusion: Only one con, but shouldn't be ignored. I recommended to buy in sales / bundle, you can see the price / hour.
Pretty sure all of you have at least heard of this game, but if you haven't, go on and BUY IT! Not only does it offer great gameplay that will make you feel like you're really holding the Portal Gun, and genial puzzles, but it also has a very good story line, to which the main AI (literally) GLaDOS adds humour and irony very needed in some of the puzzles. Good job, Valve!