Ezra Klein has a nice web piece over at the American Prospect summarizing what we know about the success of Massachusetts's universal health care scheme. The gist of it is this: It's been quite successful in reducing the ranks of the uninsured, which have been roughly cut in half (results are even better for those with incomes below 300 percent of the poverty line). There are two main drawbacks: One, while it's still early, compliance with the individual mandate hasn't been universal, and two, it's been somewhat more expensive than supporters envisioned--but only because the state had more uninsured residents than it previously realized.
Neither of these problems is very worrisome, though. On the cost front, as Ezra notes, the plan was aimed solely at expanding access, not controlling costs. If the state decides that the current plan is unaffordable absent some form of cost control (which could risk a reduction in the quality of care), that's a tradeoff it can confront in due time. Access should come first: I think most Americans would agree with Jon Cohn, not Michael Cannon, that denying coverage to the poor and sick isn't an acceptable means of cost control.
Second, at least in my opinion, it doesn't really matter that much whether the Massachusetts plan is literally universal in the sense that everyone has health insurance--the important thing is that everyone be in a financial position to reasonably afford insurance, which is now more or less the case in Massachusetts. The remaining uninsured are people who could buy insurance, but choose not to. This could become a problem from a risk-pooling standpoint if huge numbers of affluent and healthy people start deciding not to buy insurance, driving up premiums (the scenario that an individual mandate is meant to prevent), but so far it doesn't look like a major concern.
The bottom line is that the Massachusetts experiment has mostly succeeded--which is probably why, according to the polls Ezra cites, more than two-thirds of Bay Staters approve of it.