The biggest problem with Portal 2 is familiarity. Inevitably, it doesn't feel quite as magical as it did the first time you linked two portals together, chucked a cube through them and got that little buzz of achievement from completing a physically impossible puzzle. Like the first time you played an FPS, it's a thrill that can't be recreated.
The greatest success is how much it dates the previous game. Playing Portal 2 makes you realise that Portal was a series of puzzles with a story draped over the top; this uses the same basic tools but tells a specific, hilarious narrative in a detailed, (dys)functioning world.
It's all powered by relationship between the characters: the passive-aggressive murderousness of evil AI GLaDOS and the chirpy idiocy of rogue personality core Wheatley, with occasional contributions from Aperture's cigar-chewing, damn-the-expense-and-particularly-the-health-restrictions CEO Cave Johnson. The dialogue is simply exceptional throughout and it means a game based on physics puzzles has the most appealing characters you'll encounter this year.
The game takes you deep into the bowels of test-obsessed conglomorate Aperture Science, where you face brilliantly period-specific tests from the company's earliest days, and the puzzles gradually scale up in the same way as the original. Absolute beginners will quickly grasp the basics; experienced players will enjoy the new gels that increase bounciness, decrease friction, and enable previously untouchable surfaces to take portals.
The rooms have been scaled up, too. You quickly leave the poky confined spaces for vast, multi-stage expanses equipped with bottomless pits, churning meat grinders, descending spike pits and inquisitive yet murderous turrets. Thinking out of the previously-employed boxes is harder than you might think; in places it looks more like Half-Life 2 or even Bioshock, with portal locations almost hidden in the distance. For the most part it's challenging rather than difficult, although there are a few situations later on, mostly involving gels, which will irritate.
While these mechanical improvements elevate the game, the most satisfying inclusion is completely new: the co-op mode. Introduced in the main storyline but otherwise completely separate, it casts you and a friend (either using split-screen or Xbox Live) as charmingly-animated robots solving a series of puzzle rooms.
It's a similar situation to single-player, right down to GlaDOS' spite-rich commentary (here dedicated to needling each player in an attempt to make them fight) but by forcing you to collaborate it enables you to share the endorphin rush that comes when you've cracked a puzzle. This can be expressed by animations unlocked as you progress (the I've-got-your-head taunt is a particular favourite) but really, this is the sort of experience best rewarded with a high-five in person. Couch co-op has never been so rewarding.
It works almost as well on Xbox Live, although voice chat is essential. Most puzzles end up requiring a free and frank exchange of views that can't be adequately communicated using the built-in tools, which are fiddly to use in any case - although a notable problem, as with the single-player campaign, is that it's over too quickly.
Valve's stated ambition was to leave you wanting more, and in this it's just a bit too successful: while it's far bigger than the original, an experienced pair of players will still be able to burn through the entire package in a weekend. While it's worth replaying to catch the dialogue you missed the first time round - a spectacular rarity in any game - and to hunt out the hidden Rat Man rooms, there isn't enough variety in the way you solve puzzles to sustain repeat runthroughs. We're going to need DLC, and plenty of it, to keep our co-op partnership going.