Despite one major defect on the Mac, Portal 2 is an awesome continuation of the original. I gladly gave up 12-14 hours of my life over the last couple of weeks to play the single-player version. The puzzles are fun to solve. The graphics are amazing. The plot is hilarious. The music is appropriate. (One part sounds like Philip Glass, but I don't think it's his.)
Players of the original Portal will most likely LOL from the sequel's references and the way it fills in the holes. To get the most out of the sequel, I recommend playing the original first. It only has 19 levels, and you can probably beat it in a couple of hours. Note that you can buy a two pack from Steam/Valve (from the link above) where the original costs only $5 more. Plus, if you play your cards right, you may get a $60 rebate when you finish both games.
Alternatively, if you want to skip playing the original, you can watch this entertaining walkthrough. It's only two hours long.
Is the sequel worth the $50? Definitely. You get the one-player campaign, and a two-player cooperative play (I haven't played through that yet). You get more GlaDos-like humor and even another clever song by Jonathan Coulton.
A word of caution: this game is very demanding on the graphics card. Make sure to check the bottom of this page first to see if your current graphics card meets the minimum requirements. It runs fine on our less-than-one-year-old MacBook, but will not run on our two-year-old HP computer. The buzz is that this is a sore spot for people with older computers who want to play the sequel. But once you see the improved graphics, I think you'll understand.
And the Major Defect? This is only for the Mac, but evidently plug-in controllers don't work with any Valve games. Really? We're talking about a basic video game input. Even the rinky-dink freeware game emulators on the Mac use plug-in controllers. Why can't Valve figure this one out? You don't need the controllers to play the one-player campaign (as you can use keyboard/mouse), but how in the world can you play the two-player game? There's only one mouse, and both players can't share it!
Supposedly there's a way to hack into the game and get the controllers to work on a Mac. I guess that means more research. But I'd much rather that Valve fix this Defect so I don't have to.
Other than that: great game! I look forward to #3. (Please tell me #3 is coming!)