I've been taking melatonin for about ... 5 years, so if you DO have some evidence for any long term effects, that is of course something I'm interested in.

I suspect that there is some genetic variation in the circadian system such that melatonin isn't one size fits all. In particular I have a pretty long sleep onset. Before melatonin, my estimate of my own sleep estimate was about 30 minutes to an hour. With melatonin it's usually less than 30 minutes, although I've adjusted my dose down to 2 mg or so trying to get the minimum that works.

For me the benefit seems to be more than the 30 minutes saved, my sleep also becomes somewhat more effective.