I saw a series of studies once for HRT (not a surgery, but relevant to transgender research and major bodily changes) that said that 90% of patients reported either an improvement or at least no change in their quality of life after HRT compared to before. Of the 10% who reported a worse quality of life or stopped treatment, the majority of causes were due to external factors such as harassment/hate crimes or being disowned by friends and family. The least commonly reported cause was post HRT regret, and the vast majority of that 10% said that they would be restarting HRT as soon as they safely could.

Not only is that a huge success rate, but it also says something about the percentage of people who would respond to such a survey, as going "stealth" as it's referred to, can be a major component of transgender people's safety considerations. If people don't know your trans, you can't be assaulted for it. And considering the sexual assault rate for trans women in the US is 80%, they have reason to worry about that sort of thing. Also, a quick Google search tells me that the average response rate for medical surveys is 76% for in-person surveys, 65% for postal, and online surveys are 46% for website based and 51% for email surveys. So that 59% isn't too far outside the range as long as it isn't in-person surveys.