Alright, if you ask yourself "Do I need to download this mod"(Which is weird because it's free and almost everyone say it's awesome), this review is gonna show you exactly why you should.
Ok, so the plot is kind of a meh, and a bit cliche as a time travel story, but the start is amazing as you have already seen in the trailer.
It's not as long as Portal 2, but as it works on the same principle of Co-Op(Think for two players), so the difficulty would make it feel longer.
This mod is much more challanging than the actual game. Really, even Portal 1 isn't hard like some of these chambers. Lovers of challanges, you're gonna LOVE this mod.
Differences From the Base Game
Ok, almost everything is now completley retextured and looks awesome.
Overall, this is a great mod, very challanging, a bit short, and while Portals are not the main mechanic anymore, you should all play this. It's also free and you can see your legs.