Portal is terrific fun and terribly cute, but a bit forgettable, and the final "level" (running through the guts of the building) goes on for way too long - but that may be due to my aversion to dingy industrial landscapes in computer games. All that said, I know I'm going to love the shit out of it every time I play it.
I've just finished playing Episode 2, which with the exception of a couple of moments of chunking out performed admirably well on my computer (a laptop!). It starts slow with all the Antlions - which aren't my favourite enemy - but it sure picks up after that. The Hunters are incredible, an enemy that's simultaneously scary as hell and absolutely exhilarating to play against. That first fight against them is one of the most thrilling, adrenaline-pumping scenes in a game I've ever played. I also loved the "going to get the car" level, and god knows how long it must've taken them to design that map. It shows Valve's skill in taking essentially identical scenarios and twist them in different ways: the contrast in ambiance between this level and the hospital level in Episode 1 is remarkable, given that they're both basically "running through an abandoned building, killing zombies": the level in Episode 1 is basically all action, all the time, whereas the level in Episode 2 is much more tense, with occasional bursts of flash fighting. I was impressed by the way game built and built and built in intensity until that final fight - and then, of course, that final scene, which shows that when it comes to putting genuine deep emotion into games Valve has few peers.
And a final note: everyone's been talking, quite rightly, about how great and how suitable for the game the song over the end credits of Portal is. The music over the end credits of Episode 2 is just as great, in its very different way, and absolutely perfect for the way the game ends.