> Maybe all I see is people posting letters about their rates going up though. What is the argument against that from people who still support this legislation?
That what you see is not representative, because along with those I see plenty of people that could not get private insurance at any price (e.g., for whom the cost was infinite) due to preexisting conditions that can get health insurance because of the ACA.
> Is it denied that prices have gone up in general
For like benefits, costs have gone down since the ACA was introduced, in part because of parts of the ACA that went into effect earlier, like the limit on profits which required refunding excess premiums (because health insurance, even prior to the ACA, was not an ideal competitive market but an oligopolistic one in which rents were being extracted, and this was a limit on the rents that could be extracted.)
With the minimum benefit levels effective for non-grandfathered plans some low-cost, low-benefit plans are going away. This, on its own, would increase the minimum price of private insurance after this stage of implementation of the ACA above what it was prior to this stage of the implementation. In addition, changes to rules on what factors can be used in price setting have shifted costs so that, compared to what they would be without those rule changes, prices for plans for young, currently healthy individuals have gone up, while prices for everyone else have (again, considering only the effect of that rules change) gone down.
Expanding the size of risk pools as the mandate does should slightly reduce overall prices, but that may be a lagging effect since it can't be factored in fully without a clear picture of the magnitude and how many people will opt out in favor of paying the penalties for non-insurance.
Additionally, subsidies have been made available to people at the low end of the scale to which the individual mandate applies; this doesn't help with total costs (ignoring the effect that coverage vs. non-coverage has on total healthcare costs, which is significant in the long term, but not necessarily the short term) but does help with affordability.
Even moreso, except in states that opted out of it, Medicaid expansion means a lot of people are getting coverage with no out-of-pocket cost that could not afford any coverage before.
But, yes, the people that could afford insurance and who paid the least for the coverage they were getting because of youth and current health are often going to be paying more.