I liked Portal 1, but it was very short and did feel more like an experiment than a fleshed-out game. But Portal 2 overshadows the original like a mountain, it's everything the original did, done better, lengthier, more complex, detailed, funnier... overall simply better. It's still of course puzzle-based and heavily scripted however, so once you finish it there's not much replay value, in my opinion.
I have to admit though that I've played it simply because it's a first-person view game and the whole portal game-play concept was interesting, but had it been some sort of a more standardized 2D puzzle game I might have never known about it, since generally speaking I never really liked too much puzzle-solving in games (with exceptions of course, such as Zelda or older Tomb Raider games, or Soul Reaver and of course Portal). It's a different approach to "puzzle" per se, it's not the usual sort of 3D action adventure-esque "find the switch to open the tomb's door" or "carefully walk between specific floor grids to avoid triggered traps" dungeon-style puzzle solving. It's more about "listen to fun and silly comments from GLaDOS as you use a physics/"Portal Gun" to "tele-Portal" your way around a room and figure out how to move to the next.
I played and finished Portal 2, but on the 360 version thanks to my counsin's own copy, but I do not own it for the PC. I never re-played the original, which I own on Steam (it came with my Orange Box copy years ago) since for the same reasons it's very linear and has almost no replay value, unless I'd want to complete the puzzles faster or something along the lines of self-created challenges, but I don't have the interest to do that. I'd like to say that overall I had the most fun in co-operation mode which I've completed with my cousin as we laughed (and drank beer and ate pizza, what a great gaming afternoon that was) all the way from the very first moments to the end (in pretty much one sitting of a few hours).
With all this said though I'd give a solid 7/10 or 7.5/10 for the original, and easily 8/10 or 8.5/10 or so for the second, and I certainly recommend it to everyone (even those who like me never really liked puzzle solving in games, generally speaking that is). And really at $5 it's a steal since I think it's still easily worth more than that today. Again, to reiterate, that from my own experience I think that Portal 2 is best played in co-op.