Well, my own take of the immediate effects of this are:
You are going to see a number of unemployed Medical professionals, as private companies are left by their lower-band customers, and there will be a lot of complaining because jobs in a State-Funded system will not be as well paid as private practices, so you will see a separation of skill levels, the highly skilled professionals will go into the private practices, and the less qualified ones will go into the public sector (for the main part, something not dissimilar to the legal system in the UK, where the high-grades tend to go into defence work, not sure if it works the same in the US, I believe it does).
For the US, this leaves an interesting question because of the proliferation of Medical Malpractice cases that seem to appear in your courts. There's a risk here of, with lower-skilled professionals, that the number of malpractice cases against the state will be higher than against private practitioners. That will be an interesting political hurdle.
So, yes, the changeover will be rough, there's no doubt about that, however, the longer term effects of it are interesting, the requirement of extra medical facilities to deal with the new influx of patients will actually eventually cause an increase in the number of medical jobs, and encourage research into cheaper ways to produce effective treatments, since Governments inevitably buy from the cheapest bidder, and whoever gets the contract get a nice juicy income.
The main risk is bureaucratic bloat, but that's why it's important to have people who don't trust the system, or possibly, even wish to see an end to it, because they will watch it like a hawk.