I'll start off by saying this: this is the best game currently on the market that you can buy for the price. There simply is not a cheaper game that is as enjoyable. It's old, but it does not feel old. The mechanic is so unique and interesting, it doesn't need any sort of story or worldbuilding or characters or comedy or anything else to make it a great game, yet it has all of that. So lets delve into the different elements of Portal to see why it is worth the buy.
My problem with videogames often is that I get bored or am not hooked. It can be great fun, but if I have no reason to play more of it, I wont. This is where a good story comes in. Normally I am a fan of "in-your-face" story, where you have cool cutscenes and amazing visuals, but in Portal they go for the passive story approach. The clues about the world, why you are there, what happened to others, etc all can be gathered through clues around the maps. It is the hardest type of storytelling to get right, because you can't let on too much, but it has to be engaging enough to be worth paying attention to as a player. Portal is the best example of passive storytelling done right.
Comedy in a puzzle game is not something that was new, but it certainly is new in the form. It is a dry, sarcastic, morbid gladOS humor that I miss in other games. It is a signature of the franchise, and it is done absolutely perfectly.
The difficulty of the puzzles was never an issues. They never seemed impossible, yet they required some thought and sometimes trial and error. There are multiple ways to solve every room; just go watch some speedruns of the game.
In fact, the level design itself was so good that it hardly felt like I was walking into yet another puzzle, an obstacle waiting to be crossed, but rather a fun adventure of a room to test out new ideas and mechanics.
The length of this game is great as well. It never drags on, it never wears out game mechanics, but it offers other ways to challenge yourself if you so wish once the game is done.
Overall, this game defined an entire genre. If you make a puzzle game, the bar is set at Portal.
Go buy it, it is worth the buy. It's worth buying it for anyone who hasn't played it yet.