Business-wise Xbox 360's head-start definitely turned out to be the right move. I agree with some others who said that MS didn't capitalize on it as much as it could have - the pricing is still kind of high, but that's the luxury Sony afforded Microsoft by pricing the PS3 so high.
If we were to draw comparisons to Sega's early launches, there is one problem Saturn had which now affects 360 to a smaller degree: not seeing the future. Just as Saturn was not being designed for 3D and had to have it shoehorned in, 360 has not been designed for the casual market and now MS is trying to catch up with Nintendo.
Edit: But MS couldn't afford to gamble on a Wii-like system when the Sony set-top box was perceived as a big threat to the Windows market. Since beating PS3 was the top priority, the head-start on a "traditional" system was the right move. It has put 360 in what I think is an insurmountable lead versus the PS3, and it's in a strong enough position that it's still a very attractive platform to publish games on. As HappySquirrel said, it would be hard to justify developing for PS3 and 360 if all three had launched at the same time, since Wii would likely have over half of the market!
