The plan is here if you haven't seen it. Aside from disagreements over how much health paternalism the government sound engage in to keep down costs (Brad favors A LOT), the funding scheme also raises some issues.
First off, 20% of your income can be a lot of money depending on your situation. This doesn't matter that much for people who do have insurance who will recoup most of the cost of their previous insurance plans. However, this will be a major drain on the liquidity of those who don't currently have insurance who will have to fork over 20% of their current income up front. Three quarters of this (15% of total) goes into a HSA, but for someone who is unlikely to need much non-emergency care (young males) or will be mostly covered by the free preventative care (young females), this money is effectively tied up until the next tax rebate.
For most people who have insurance, it will be a step down in coverage. This may actually drive savings by making costs more transparent to the consumers, but on the other hand, it means that there will still be an unsatisfied demand for insurance due to risk adversity (15% of most people's income is a lot for them), so unless it is outlawed, the demand for some private insurance will still be out there. I'd also like to point out that insurance companies are probably more efficient drivers of cost suppression than individuals, since assessing medical necessity requires specialized knowledge and they both have a financial incentive.
Finally, health care spending, the distribution of which is neatly displayed in Figure 1 here is dominated by the top quintile, which accounts for 80% of spending. Accounting for a mere 3.4% of costs, the bottom 50% would mostly be using just the preventative care and will have a bunch of money pointlessly locked up in an HSA. On the other hand, the top 5%, where 49.2% of the costs lie, will tend to be well past the maximum out of pocket. The area where the least cost-effective interventions lie is also where government rationing is going to be the dominant factor, which you may take in whatever direction your presumably strong preexisting opinion of the efficiency of government health care rationing guides you. This is also unlikely to be covered by the 5% of income allocated for both it and the free preventative care (aside: Free preventative care isn't that expensive and probably isn't a horrible idea if you're going single payer. Of course, insurance companies tend to cover it well too, so there's nothing special about single payer here either.), so we'd be on the hook for some general budget tax increases on top of the 5-20% extra on income.