I'm of the opinion that sex reassignment is a mistake. I just think that gender stereotypes are BS anyways, and that if men were allowed to be 'sissies' and women to be tom boys (which they now largely are) then such a physical change would not be needed. However, despite feeling strongly that it's not a good idea, and won't really solve the issues that it's supposed to help deal with, I don't think that's a decision that anyone can make for anybody but themselves.
I will also say that I have a real issue with some parents who are 'helping' their young children (as early as age 5) by referring to them bu their preferred gender and agreeing to get them hormone therapy so that they develop physically towards the desired gender's body type. You know, I have an ass and a little bit of hips, and I've seen plenty of women without much in the way of curves. Am I less of a men or are they less of a woman because they don't have the 'correct' body type and shape? This whole thing seems so much about changing to conform to society's gender stereotypes rather than overcoming gender. It really makes me sick, but then again, do I want the government to step in and tell parents that they can't (with the consent of their kids) go down that road? No. Just like how I hate that some parents teach their kids BS creationism, but there's no way to stop it without endorsing state run "thought correction" programs.
When my friend Sophia started with the hormones she got disowned by her family. She was very scared about telling a lot of her old friends (I'm one of the few she still has). She asked me once if I supported her decision and I told her, "I think you made a mistake. I don't understand it. I don't agree with it, but I feel the same way about Chris studying philosophy. So don't worry, we're still friends so long as you still want to be."