Just based on my own little support-group family of girls in my city who all started around the same time at the same age:
-1 never made it. He got NO results, even after 6 years of HRT and an orchiectomy, and is still living as male despite still being on HRT.
-3 successfuly transitioned and are plenty passable, but are still unemployed, chronically depressed, and still constantly bringing up suicide. I still worry a lot about them. (And one of them is still this way despite having already had SRS.)
-2 are in the camp of being relatively okay with themselves but still having a lot of depressive bouts and still trying to figure out who they are, (probably because they're still dealing with serious family issues, plus passability issues, and one was dumped by her fiance for "not being mature enough" and has been questioning herself and her gender ever since.)
-5 transitioned more or less 100% successfully, and despite losing a lot career-wise due to transition, are now working new jobs as themselves, more or less completely passable, working toward SRS, and with relatively few bad days. (And 3 of them are even in stable long-term relationships!)
-1 finally did transition successfully and happily, but has a long past history of unrest, of transitioning and detransitioning multiple times before she finally settled in.
So, out of 12 of us, only three have had identity-questioning issues, with one detransitioning and retransitioning, one ending up adopting a more genderqueer identity, and one never making it. The rest are ALL still living their lives as a girls, and pretty much unwavering in regards to their identity. But despite that unwavering identity, about half of them are still struggling with depression, image issues, family issues, employment issues, romance issues, just always some issue making them depressed despite being post-transition and more or less completely passable.
So in terms of "success," as in actually making the transition and living life in that new gender role, I'd say that detransition or not ever making it to being one's identity gender seems to be pretty rare. Usually once someone is actually on hormones, they know pretty quickly that they can't go back. Even the ones with the most trouble NEVER say "I want to be male again." All of the depressed ones are more just wishing they could be cis girls. And yeah... true acceptance of their new gender role, and acceptance of being trans, and actually being happy and comfortable and a more or less normal self-assured person with few emotional issues related to being trans, seems to be about a 50/50 tossup. (And even my "successful" friends still have their days. It's just that those days are more rare for them, where the "unsuccessful" ones are constantly complaining about their issues.)
(To be fair, though, this is a small sample size, and it's all a group of MtF girls who transitioned between the ages of 25 and 35, so it's admittedly a very small subset of the trans community. I have no idea what the "success" rate would be for FtMs, younger high-school/college transitioners, or older (35+) transitioners. I don't have many friends from any of these subsets, and thus don't know enough about them to diagnose their mental states.)
