The Orange Box includes just about everything - including cake. If you live to eat it...
For me (and many others), Portal is what makes The Orange Box stand out as a great buy. The portal gun is an awesome toy, and brings an increasingly rare breath of originality to the world of video games. But the portal gun, and the great little puzzles it is used for, aren't enough to make Portal great. Everything, from the environment (I am sure there is a metaphor involved with the dingy dank underside of the pristine laboratory, and thank you Valve for not beating us over the head with it), to the personalities (I will truly miss you, Companion Cube), to the audio (I looped Still Alive for 2 hours on my laptop and never got sick of it) combine to make Portal truly shine. Sure, the game is a little short, but the $60 price tag buys you a lot more than just Portal, and I don't know if much could have been done to lengthen the game without ruining it.
Team Fortress 2 is good as well. At least, I think it is. My one attempt to play it landed me on an extremely laggy host, and I spent most of my time running into a wall and shooting at the ceiling. The overall design of the game seems rock solid though, and would probably be a lot of fun in a situation where you could actually play it.
I have to admit to not spending much time with Half-Life 2 itself, and none at all with its companion chapters. I tried to get into HL2 again, having played it on the PC, but I feel like the game is really starting to show its age (even though it's not all that old). The graphics, the collision detection (which is still too good - I hate getting stuck in doorways as a consequence of not hitting them dead on), and most of all the story pacing just aren't enough to keep me engaged. Maybe games like Halo 3 and Call of Duty 4 are giving me a bit of ADD...
In any event, it's hard to imagine many people picking up The Orange Box and not winding up feeling like they got more than they paid for - and that makes The Orange Box a solid buy.
As for the cake... play the game.