Every once in a while you'll run into a game you just love despite a glaring fault. This is that kind of game.
Portal is set in the Half Life universe, and beyond that and the engine used to make the game, it shares virtually nothing in common with HL2. Instead, for the entire game you are trapped within a research facility, forced to solve various puzzles to make your way forward. Aiding you in this task is your portal gun, an awesome tool that creates doorways between two positions in the game world.
The portal gun I believe is probably one of the biggest innovations in an FPS I've encountered in quite some time, and it makes for some real mind bending possibilities. You'll use this handy little tool to defeat turrets, move cube, and generally avoid danger, and it all feels so very natural that you can tell the game designers really put some real good thought into making it work well.
However, in the end, it isn't the portals that makes it such an endearing game. No, what makes portal so very grabbing is GLaDos, your AI tormentor. Throughout your journey in the facility, GLaDos will speak to you, with a very computer like, but also strangely twisted mind that becomes ever more evident as you proceed. Add in the subtle elements of story from your previous test victims' scrawls on the walls, and the gun turrets with child like voices, and you get such a marvelous experience in a game that few have match.
So, what is that glaring fault I mentioned, it's length. Portal clocks in at maybe at most 3 hours your 1st time through, and quickly drops from there on subsequent playthroughs (I did my last one in just over an hour). There are challenge maps that add to the time, and they can be fun, but it just doesn't match up to the wonderful single player experience.
At $20 dollars (it's individual price) or as a pack in with orange box, it worth it I think either way, and just such a unique experience that I feel everyone should at least give it a try via rental, or a friends house.
Oh yes, and the cake is a lie! :)