I was at Magen's Bay the first time 25 years ago on a vacation to St. Thomas.
Even though our family routinely vacations on nearby St. John, I hadn't returned to Magen's Bay until my 19 year old son and I cruised together in March and went on the Magen's Bay shore excursion.
It was great. We got there about 10:00 a.m., well before it got crowded. I strongly recommend that you eat lunch there very early (think 11:00 a.m.), as there is a slow, hot, crushing line later.
Our entire family, plus my son's girlfriend, went to Magen's Bay in June, while we were vacationing on St. John. It was still worth going, as my wife hadn't been in 25 years and my daughter and son's girlfriend had never been. But, it was very crowded by the time we got there.
If you are going to go to Magen's Bay, get there early and eat early.