If you liked the first one, you'll love the second one.
It is everything the first one was, but with more. More hilarious dialogue from more characters besides just Glados. More Portal puzzles, with more elements that all function quite differently and each add new dimensions. More play time (It has to be at least twice the length of the first), more depth into the background and workings of Aperture, more areas, more everything. For what it is worth the game also looks much nicer, environments have so much more going on and it hammers into you the feeling of just how Massive the lab is.
Co-op is great too, it alone easily matches the length of the first game and is filled with hilarity. Make sure you play with someone you can actually well.. cooperate with though.
There is not a thing I could think of that was not, at the very least, on par with the first game.