The good thing about the Orange Box is that you can gift duplicates of what you already have. You said that you have HL:2. You should (theoretically) be able to gift the copy in the Orange Box to another of your Steam friends. It's generally like that with all Valve games/bundles.
As far as its worth, 3 words should sum it up:
YES.
YES.
YES.
I've replayed Portal over 2 dozen times now. It's STILL fun every time, no matter how fast I do it.
Half Life 2 is by far the best game I have EVER played. If your computer can handle it, check out FakeFactory's Cinematic Mod v10. If you google just that, you should be able to find it along with a torrent link. Beware, it's about 10gb. However, everything is retextured, reskinned, and they have included new cinematic music. It's absolutely stunning.
Team Fortress 2 is by far the most fun FPS I've ever played. The graphical style is so different, and the game itself is so quirky ... it's impossible not to enjoy it.