We were just there while attending Tanglewood. It's definitely operational and both Saturday and Sunday afternoon around 4 to 5 we were the only Tesla there. This was the final Tanglewood concert for the season, so I expected there to be other Teslas charging, but there weren't. BTW, if you are at all into classical music, make an effort to see Andris Nelsons conducting, he is really brings out the best of the BSO.
The Big Y actually has some decent ready to eat food offerings and a restroom. There doesn't seem to be any place to sit down though except your car.