Old games? Sure. But if you haven't bought this product yet, you're missing out on a great deal.
As Source is central feature of all of these games, they all have similar characteristics that make each one of them fun. Let's start with Half Life.
At the time it came out, Half Life 2 was revolutionary. Today, years later, it isn't as graphically advanced as current games, but is still a variation from your average FPS and definitely something every PC gamer should play. The gameplay is constant - there is absolutely no break in the action, save one (which your character doesn't even notice). As you play through the arenas of City 17, Nova Prospekt, and other memorable locations, you face a diversity of enemies and an even greater diversity of ways to kill them (where do you keep all of those weapons, anyways?). The graphics of Half-Life 2, old as they are, are still good in comparison to modern games. In way of errors, I found only one skybox error in the entire game.
Half-Life 2: Episode 1 is probably the least memorable of the series, but it's still fun. It is by no means long, and not many people use the Ep1 engine to map, but it's obviously crucial to understanding the story and still sports fun combat from dark garages to the Combine Citadel.
Episode 2 is arguably the crowning achievement of the Half-Life series, brought down mainly by the fact that it only lasts a few hours. In this game, Valve showcases their upgraded cinematic graphics (the game even starts off with an entire bridge being destroyed in an expertly designed manner, especially for the year it was made). The landscapes in Episode 2 are also a central feature of the game, which are both expansive and beautiful. An incredible amount of detail was put into making your adventure to White Forest beautiful and enjoyable.
Overall, the Half-Life 2 cycle's storyline is great. It already starts with a good basis - a commonly used, yet still immersive story about an extra-universal alien alliance taking over Earth. There are moments in the story where you feel completely epic, and moments where you are almost brought to tears. The story line, as a matter of fact, is something I anticipate even more in Episode 3/Half-Life 3 than the gameplay (assuming it ever comes out...). These three games are enough by themselves to merit buying the Orange Box, but two more masterpieces make it the best buy in the industry.
Portal is about as much fun (and humor) as you'll ever have in a puzzle game (save for it's sequel). The challenging, but not annoyingly mind-numbing challenges, the hilariously evil character of GLaDOS, and the revolutionary use of portals to complete puzzles made it's sequel one of, if not the, most anticipated game of 2011.
Team Fortress 2 is one of the most popular multiplayer shooters today, even though it was released years ago. The revolutionary use of multiple diverse classes, upgradeable equipment, hilarious characters, and addicting gameplay and game modes makes Team Fortress 2 a game that has yet to be completely capped in addictiveness and fun.
As if the base games themselves weren't enough, every Source game can be modded and can (and has) had custom maps made for it. So you'll never be out of diversity in maps and gameplay. All in all, any three (or even two) of these games fills the price tag perfectly. Getting all of them makes you crazy not to buy it. So buy it.