Chances for keeping insurance premiums down and getting many more people enrolled under new Affordable Care Act policies this year are looking good, according to a new survey. The survey found that about 1 in 4 adults visited an Obamacare exchange by the end of December. And more significantly, the vast majority of those who visited and are potentially eligible for coverage there—a total of 77 percent of those surveyed—reported being in either excellent, very good or good health, according to a statistical snapshot taken by the Commonwealth Fund. And they were not disproportionately old as has been the case in the limited demographic data released by the Obamacare health exchanges to date. In fact, 41 percent of the exchange visitors were 19- to 34-years-old, tracking their share of the general population of people eligible for coverage via the exchanges in either private plans or Medicaid, according to the Commonwealth Fund. If those trends in health status and age hold up among the people who actually enrolled—and who will ultimately enroll—it increases the likelihood that insurers will not have to raise premium prices excessively next year to compensate for a bad risk pool that comes from having too many sick and old people signed up.

As the survey noted, "Young adults are disproportionately likely to be uninsured, and their participation is critical for balancing risk and helping stabilize growth in health plan premiums over time." (Read more: Obamacare first-person: Real people, real stories)

"I think the distribution of both age and health is very encouraging in terms of having a well-balanced risk pool," said Sara Collins, lead researcher for the survey and vice president of health care coverage and access at the Commonwealth Fund. The survey also found that a solid majority, 59 percent, of people intend to enroll in Obamacare exchange policies or check their eligibility for government subsidies to help them pay for those plans by the open enrollment deadline on March 31, if they haven't already done so by now. Encouraging signs "Overall, these are very positive findings," Collins said of the survey, which questioned 662 people ages 19 to 64 who are either uninsured or had individual private insurance. But there was a potentially big caveat in the survey: Just 36 percent of respondents reported that it was very or somewhat easy to find a plan with the coverage they needed. And only 37 percent said it was very or somewhat easy to find a plan they could afford. (Read more: Down on Obamacare, and feeling light in the wallet?)

The survey did not ask them whether they had successfully enrolled for Obamacare coverage after visiting the exchanges. "Adults who are potentially eligible to buy health insurance through the marketplaces are demonstrating their determination to gain health insurance this year," said Collins. "It is encouraging to see a greater share of those who could get coverage visiting the marketplaces, particularly the large share of visits by young adults and people in good health, since their participation will be critical to the marketplaces' success over time," she said. "I think this will alleviate some of the concerns that young adults are not going to participate in the marketplaces." And, noting the large percentage of people who say they intend to enroll in coverage in coming months, Collins said, "They bode very well for the numbers of people who may enroll in the next three months."

As of Jan. 1, about 2.1 million people had selected a private insurance plan sold on either the federally run marketplace, HealthCare.gov, or on of the 15 such exchanges operated by individual states, according to federal officials. Almost 4 million people by Jan. 1 had been determined eligible for Medicaid via those exchanges. Health and Human Services Department spokeswoman Joanne Peters said the administration is "thrilled" 2.1 million Americans signed up for marketplace coverage in the first three months of open enrollment. "We know that there is strong interest in the product and we expect that enrollment will continue to grow," she said. Data on how many of those people who selected private Obamacare plans had actually paid their first month of premiums, which is required to formally enroll, is not available. Also not released yet is a breakdown of national enrollment by demographic categories including age, which would shed light on the potential quality of the risk pools. (Read more: Mum's the word on Obamacare demographics) The national signup total so far in private, exchange-sold insurance is far short of the 3.3 million federal officials had originally predicted would be enrolled by that point. But the actual results were widely viewed as fairly healthy, taking into account the disastrous first month of operation due to website flaws and ongoing problems in several state-run exchanges. An earlier Commonwealth Fund survey found that 17 percent of adults potentially eligible for Obamacare coverage had visited the federal or state exchanges in October, that first month of operation. By the end of December, that number had grown, with 24 percent of adults eligible for coverage having visited an exchange.

Collins said the growth in visits was encouraging, because it suggested that many of the visitors who had been stymied by HealthCare.gov's tech troubles in October made efforts to revisit that and other sites once those troubles were sorted out. However, 69 percent of the visitors to the Obamacare exchanges by December rated their experience either just fair or poor—a result that essentially mirrored October's. But the survey also found that by December, 51 percent of visitors to the Obamacare exchanges said it was at least somewhat easy or very easy to compare the costs of premiums for plans being sold, while just 37 percent gave such answers in October.