Well Halo is easily Microsoft's flagship game series. It's an incredible game series though and they are lucky to have it exclusively.
I don't agree with people who say that Halo is the only reason to buy an XBOX though. XBOX Live is a great setup and it's the best out of all of the online console setups I've seen so far.
I don't see XBOX going anywhere bad anytime soon. Right now they are easily the #2 gaming console on the market, and in many ways they are hitting a different market than the Wii with their much broader selection of Mature games. They are different enough that a lot of people are simply buying both of them as well.