OK finished the game. Honestly as the end approached it became pure torture and I had to force myself to finish it. It wasn't that hard, but it was just totally un-fun. And I was constantly distracted by better games that are more fun including Valve's own TF2, travelling & other non-PC activities (so you don't think I am stupid, although you may well might

)
End credits are probably the best part of the game.
Puzzles are not too hard, but they reminded my why I hate puzzl-ish parts in HL2, and why IMO puzzles don't really belong to a 3D FPS-ish games. OK I know, you all disagree, whole world disagrees, and probably all game reviewers disagree.... I am a grown man I can live with that
One thing Valve should be praised for. They probably milked what is really weak concept for all it's worth. They actually managed to build a story, roles, and whole virtual world around boring puzzles that most people play on Flash game sites. It actually works! It's not the "best game ever" but considering how weak was the material they had to work with it works surprisingly good.
I rated the game 74% in the mag I occassionally write reviews for. Nice game, fairly priced, fun for couple hours, but I still think my rating is the lowest any reviewer ever awarded to the game
(Do I get a CAKE for that achievement? LOL)