New Jersey's attempt at healthcare reform, which lacked a mandate, didn't pan out very well:
[T]he mix of people left in the program changed dramatically. According to a study published in Health Affairs, the median age for enrollees jumped from 41.9 years to 48.4 years in just five years, and premiums rose by between 48 percent and 155 percent, depending on the plan.
What's likely to happen if the Supreme Court strikes down the mandate but leaves the rest of the law intact:
[Massachusetts Institute of Technology economist Jonathan Gruber] has suggested that removing the mandate from the law would diminish the number of newly insured by nearly two-thirds and raise premiums overall by 30 percent.