THIS REVIEW CONTAINS SPOILERS. read at your own risk.
I, like every other person who wasn't a godless Communist, loved Portal. I loved its bleak, uncaring-bureaucracy humor, the always-entertaining manaical AI GLaDOS, and the uniqueness of the main gameplay. As such, I was a bit concerned that some of it might be lost in the transition from '3-hour glorified tech demo' to 'full-on, Triple-A release'. Thankfully, it now feels more robust, more polished, and more epic in scope. The style of humor shifts throughout the game from pitch-black uncaring corporate-speak at the start, to GLaDOS's passive-aggressive axe-grinding, to Cave Johnson's can-do, optimistic, charismatic, yet utterly psychopathic persona, to Wheatley's laughable attempts to be a menacing villain. It's also fascinating to see Aperture's progression throughout the years, from a bright, idealistic company testing on astronauts, olympians, and war heroes, to a bankrupt shell testing on its own employees (presumably by force) and replacing them with robots. Both J.K. Simmons and Stephen Merchant nail their roles as Cave Johnson and Wheatley respectively, providing superb portrayals of an insane CEO and a chipper idiot.
The mechanics are also nicely varied; new paints that affect the nature of the tests, lasers, light bridges, excursion funnels, and more. While the tests for the most part are a bit on the easy side, they're generally fun to complete even if you know precisely what to do; it irons out one of Portal's few issues, the occasional segment that relies on unreasonable timing/dexterity. In Portal 2's tests, it's more akin to setting up the steps of a Rube Goldberg machine and putting it into action. However, the gels can be a bit fiddly at times, and for a few puzzles the solution is literally just 'coat every surface in sight', which seems a little bit like phoning it in on the puzzle-design front.
The co-op is a great bit of fun to play with a friend; while the single-player is mostly sightseeing and story with light puzzles, co-op is mostly puzzles with minimal story. Still, the story works well enough as a framing device, and the puzzles's solutions are quite satisfying to figure out.
All in all, Portal 2 has clever puzzles, pulls off having an epic scope, and it's a hilarious game that isn't just 'hilarious for a game'. Despite whatever minor quibbles I might have, it's still definitely worth your time.