I made the transition pretty poorly.
My main health care from age 14 (just a week after diagnosis) to age 20 or so was the pediatric endocrinologists at a very well known and respected university teaching hospital in my home state. The completely coordinated all my healh care visits (including opthamologist and other specialties). I felt very well served by them and got to participate in many research studies and "help out" with a lot of the pediatric support groups. This was back in the 1980's so of course the technology was kinda primitive compared to today, but the pediatric endocrinologists were very progressive and pushing a lot of the DCCT goals including home bg monitoring and dose adjustments, well before any official DCCT results came out.
At age 20 I transitioned to the "adult endocrinology" group at the same university hospital and was underwhelmed. The standards of care were so much lower than what I was used to - they were surprised that I was doing home bg monitoring and aghast that I would be changing my own doses.
At age 22 I moved across the country for grad school and again was underwhelmed by the adult endos I got to visit. At this point I mostly fell out of the professional health care system for more than a decade and just continued what I had been taught as a kid.
Finally in my 30's I had a real job and got to visit some doctors, and eventually settled on a really good doctor of internal medicine (not quite an endo) that I see every few months, and I got back to seeing opthamologists etc. I've tested the water with a few endos but haven't found any who are a good match to me. Not that they're bad endos, just that they don't really have time to discuss the fine points that I want to discuss.