Before diagnosis, I researched for several months before telling anyone. I wanted to be sure it couldn't be anything else. The first person I told was my GP and I had no idea what to expect. He didn't seem to have any doubt I was correct and was supportive in obtaining assessments and readily made referrals. Going by other accounts, I was d*mn lucky. However, the services aren't there after diagnosis and we can't do much about that, though are still working on it.
Some people heard about my AS a couple of years ago and at first couldn't understand how high functioning autism could exist. One told me "You're not autistic ." However, as soon as they'd done some reading, they came round to the idea and thought it explained so much of my past behaviour. However, they also pitied me and how my life had been, which felt very uncomfortable. I wanted to keep the diagnosis confidential, but one of them later told nearly everyone they knew who had, or was about to, come into contact with me. They also told me the reactions they'd received, which were generally "Oh, that explains that time when ..." or dripping with pity. Not very pleasant but, since I had little contact with them, I didn't lose much.
I informed one person from my first university who contacted me via email out of the blue one day. Their reaction was to reassure me of all my great qualities and not comment much beyond that. I suspect they didn't really believe it.
Overall, it's been positive. People understand me better and don't automatically assume I'm being intentionally rude.