I am no lawyer so I have no great insight about the legal side, but as a practical economics matter I didn't see how the mandate was much different than a tax. The government taxes and provides lots of stuff whether we want it or not - this seemed quite similar to me. Apparently the Supreme Court thought along the same lines. As an economist, however, I did recognize the importance of having a mandate in place as a part of the overall healthcare reform effort.
But as an aside, I am pretty glad that it appears we can have justices both conservative and liberal that are independent thinkers and are not subject to political pressure.