What abt plagues of black flies in that part of Maine in May? I guess sometimes they're bad, sometimes not so?

I've done the drive from Boston to Acadia in May and it was definitely worth it! It is a beautiful park and Bar Harbor is very close to it so you don't have to spend a lot of time getting back and forth. May seems to be just before the tourists start piling so we didn't have any problems at all with dealing with a lot of people yet everything we wanted to do was "open". We got lodging where we wanted, never waited in line for restaurants, etc. As already mentioned, the weather could go either way. One day we had rain and gray skies all day and the very next the sky was blue with sunshine. The only down side for us was that some of the trees were not yet blooming so in some spots it still looked winter-like instead of spring but that was the case for most of New England that year. I think July/August is when the flies usually are the biggest problem.

How beautiful is Acadia?
Bar Harbor & Acacia National Park have visitors from all over the world as well as from all over America.
I have met Danes, Germans, and French Nationals who have all made the trip specifically to see New England and Acadia.
So if it's worth that kind of travel for them to see it, a 6 hour car ride is a piece of cake.