From someone that didn't play the Orange Box version.
But from there, is where the adventure starts. I don't want to ruin anything but if you haven't played this game, if you can handle FPS's (without getting sick) this is by far one of the best puzzle games to date. I would put it up there with Tetris, but not the same TYPE of game.
Even after you completely the 1-player game and get your cake (yes you actually get it, unlike the Orange Box version), there are a host of challenge maps that reward you for 3 different areas. The amount of portals used, the amount of steps taken, and the time taken to finish the level. Eacho one has a certain goal in which you can get a Bronze, Silver or Gold medal.
To me, that simple feature adds a much longer and indepth replay value on this amazing title. But I am one of those that wants to prove that I could do it better then the rest:P
In closing, if you havent played this game, it is a MUST for all XB360/PC owners. Plus you get a kick arse song to listen to after completing the game!