Throughout the single-player campaign I was constantly surprised how clever you could use this "simple" formula of two portals, and I admit, I had to look one or two times in a walkthrough - still, it's manageable for others like me who don't solve puzzles often and I really liked the feeling of accompishment after solving a puzzle and the reward of getting a new piece of the (mostly silly and amusing) story ("lemonades").
And the coop-campaign: I don't know any other game where you solve puzzles this way. Really, it's a lot of fun and shouldn't be missed out either (if you have someone to play with). It was also one of the first games on Steam with Splitscreen support. Although it's only for two-player, I'm pretty sure that it also would be funny with three or four people locally, when everyone's trying to figure a puzzle out together and the gamepads get handed around.
Lastly, there's the Workshop integration, and boy oh boy, are there a lot of puzzles. There are very complex ones with custom terrain and more simpler ones, created with the later released simpler editor, and there's an endless supply of new ones. I acctually recommend trying the simpler editor out yourself, just to see how hard it is to come up with puzzles.
Anyways, this is a game that everyone should play, even if it's not your genre. Because this is one of those defining games others will draw inspirations from and have to be measured by for years to come. Also, it won't get a sequel, because 3.