The original Lego Star Wars was an unprecedented success. Who'd have thought that a game where the you played through the prequel episodes entirely made of Legos would've been such a huge success? Lego Star Wars II: The Original Trilogy soon followed and was overall a better game because they were based off better movies (and some of the gameplay had been refined to be better). The Complete Saga now sees both games put together in one package. You can now play through all six episodes without any interruption. There are new levels to explore, the refined gameplay of the second game is used all throughout and that also means there are more characters.
You'll play through the saga's best lived moments, and there's a multiplayer aspect thrown in for good measure. You and a friend will conquer all six episodes by solving puzzles and using the force. The game is filled to brim with a great sense of humor and you'll be drowned in nostalgia as you play through episodes IV, V and VI. There's nothing to difficult about the gameplay either. You'll take down the bad guys and solve puzzles to open doors. You'll build things to help you out and pilot vehicles like the Pod Racer from Episode I and the X-Wing. You'll also collect studs to by many of the games unlockables including characters and vehicles. Then, of course, there's the customization that'll let you make a custom character. And with well over 100 characters, there are literally thousands of possibilities.
The only real big problem with Lego Star Wars is simply that the game is way too easy. There's no such thing as a game over. Should you die, the only bad thing that happens to you is you lose some studs. At least the game is so much fun that you really don't care. Also, while cooperating with a friend, you can both battle each other and to the victor go the spoils. Its great fun even if you can't die permanently.
This is great for someone who has never played the Lego Star Wars games before, but what about if you have? There's really not a whole lot of new content here for those who played the first two games to begin with. Some levels have been altered, but for the most part, it's all the same stuff. True there are a bunch of new levels, but if you've already got the first two games... the list price just isn't worth it to play what will virtually be the same two games all over again. Even if there are some extras, it's just too much of the same thing.
This then, brings about the question as to whether or not it would just be better to find used copies of the first two games. That really depends. The XBOX360 version has better graphics. It's smoother and runs better. If you buy the original games separate and used, you'll have to downgrade to the PS2, Gamecube or XBOX to have both (since the first game didn't come to 360 at all). The simple solution is this: If you have one, but not the other, it's easier (and cheaper) just to go pick up the game you don't have. If you have neither of them then by all means pick up The Complete Saga.