An upgrade in story, but a downgrade in puzzles
Portal 2 continues where the first game left off without any fundamental changes. The players finds himself back in the testing facility and has to do through a bunch of test chambers again. As an unspecified amount of years has passed since the first game, the facility is no longer in pristine conditions and a bit of vegetation and defects make it look more interesting. GLaDOS makes a return and is joined by another AI by the name of Wheatley and Cave Johnson, the Aperture Science CEO. The back and forth between the character makes for a more interesting and dynamic story then in the first Portal, the overall plot however still feels kind of meaningless and random, as at the end of it little more then a bunch of test chambers and funny one liner happened.
The puzzles in Portal 2 feel a bit like a downgrade. The first hour is essentially spend redoing puzzles already seen in the first game and most of what follows later on isn't all to difficult either. The game does introduce some gels later on, a blue one that allows you to bounce and a orange one that speeds you up, but they don't change the game fundamentally and their use in puzzles stays rather simplistic.
The biggest problem with the game is that the puzzles don't properly integrate with the story. In essence the game has two modes, either you go from test chamber to test chamber and trying to reach the exist, which can get a little old after a while, or you are moving around outside of the test chambers and trying to reach a story related goal. The puzzle design in those outside section is however close to non-existent. Almost everything you do in those section boils down to simply finding a tiny white wall in the distance that can hold your portal and then teleporting yourself over there to move forward. There is no thinking or clever combination of talents, it's essentially just a game of "Where is Waldo".
On the technical side of things the Source engine shows it's age. While the graphic still look quite nice, the loading times are horrible. Not only are they rather long, they are also extremely frequent. And any time you die, which luckily doesn't happen that often, you have to sit through them again. Given the otherwise seamless nature of the game, those loading screen really feel rather annoying, especially since they are now handled by actual full loading screens, not just tiny "Loading..." text prompt over. This is especially annoying given that the game is full of elevator rides that would have given plenty of opportunity to hide the loading.
Overall Portal 2 doesn't feel like either a clear upgrade or downgrade compared to the first. The setting and characters are more interesting and the additional length with around 7 hours is welcome, but the novelity is no longer there and many of the puzzles just feel flat out uninspired. The game also still holds far to tight to it's puzzle game origin and never confronts the player with challenges he can solve in a dynamic way, it always feels very rigid with there always being only one true solution to any challenge. This also manifests itself in a lack of environmental interaction that can sometimes end up rather confusing and frustrating. At one point in the game you have to replace a turret with a different kind of object, and while there are plenty of chairs near by, you can't grab any of them, you can't interact with them, and a coffee cup, one of the tiny few objects that you can grab, won't actually solve the puzzle. The game forces you to follow the one proper solution, instead of allowing you to improvise. This also manifests in the navigation, you always have to use the portal gun, just duck jumping through a broken window is not allowed, even so the hole looks big enough. Smashing glass isn't allowed either, only when it will only break when it's a prescripted story moment. There is lots of moments in the game that would have lend themselves just fine to giving the player some freedom to smash and improvise, but Portal 2 just doesn't allow it.
In the end it's a solid game, but not much more then that. The story is to much driven by funny one liners, not by substance and the puzzles aren't all that interesting either.
This review is for single-player only, haven't tried the coop-multiplayer.