I know I'm late to the party, but I picked up the Orange Box for $30 last month and tore through Portal in one sitting. What a great game.
Portal's gameplay is pretty straightforward: you're given a gun that can create two portals which allow you to traverse sections of rooms. It's a concept so simple, in fact, that it's hard to envision an entire game built around it, which is precisely why I find Portal so impressive. Despite its simplicity, I never felt bored, and the puzzles continued to surprise me all the way to the end.
The quirky running monologue from the AI character and the brilliant end credits really round out the game, too. It's easy to imagine these details being neglected, but the game is much richer for their presence and quality. Would people still talk so much about Portal had they not discovered that the cake was a lie? I doubt it.
For what it's worth, I've barely touched the rest of Orange Box. Don't let the short length - about 4 hours total - fool you: Portal alone is worth the cost of the Orange Box, and it's the best game I've played in the last few years. HL2 and TF2 are just neat little bonuses.