Normally this would be a lengthy entry, but given that there have already been 4 other major national surveys saying the same thing, the latest news is just locking things down.
The New England Journal of Medicine just published a new report which concludes (shocker) that there's been a net reduction in the number of uninsured U.S. adults by around 10.3 million since the ACA exchanges went into effect:
As compared with the baseline trend, the uninsured rate declined by 5.2 percentage points by the second quarter of 2014, a 26% relative decline from the 2012–2013 period. Combined with 2014 Census estimates of 198 million adults 18 to 64 years of age,19 this corresponds to 10.3 million adults gaining coverage, although depending on the model and confidence intervals, our sensitivity analyses imply a wide range from 7.3 to 17.2 million adults.
They go on to note that this is right in line with numerous other national surveys/studies by the RAND Corp, Gallup, the Urban Institute and the Commonwealth Fund.
In addition, like those other surveys, the NEJM study does not include children under 18, who make up around 23% of the population. That's around 72 million children; even accounting for the fact that kids are more likely to already be covered, you can almost certainly add another 2 million or so to the 10.3 million adults (2M out of 72M is just 2.7%, vs. the 5% or so drop in uncovered adults which the various surveys have found). Heck, even if you assume just 700,000 children nationally (less than 1% of them), that would still bring the total net reduction in uninsured to at least 11 million. The other 4 studies range from 8 - 10 million adults, so somewhere between 11-13 million total seems certain at this point.