I forget -- are there 18 or 19 levels in Portal? In any case, the last level is about long as the rest combined so it might work out to be a tetch longer than your guess. It's definitely doable in three - four hours though. The advanced challenges are also fun, and the developer's commentary isn't bad either!

Plus the ending is so satisfying.

That's a really good game -- it actually won several game of the year awards over bigger, fuller experiences like Halo 3 and Bioshock. I think it earned it too. Honestly, there's no reason anybody who's interested in making small games SHOULDN'T play through it.

I'm fired up for them to publish some extra levels for it, but given Valve's plodding release schedule, who knows when that'll be?

I certainly loves me some Rock Band. I got tired of the janky guitar and ended up buying GH3 (which I already owned, for PS2) just to get the guitar. How sad is that? Some of the extra tracks are really good, too. Plus we play co-op a decent amount it's really fun -- definitely the best way to play the game.