I have taken it on 34th st at Penn Plaza twice, the two centers are in the same building, on different floors...
I think the center is great, never gave me any trouble, I took it on the 17th floor both times I took the MCAT, so there wasn't any problems with distractions from outside,
I once registered for the other prometric center 635 I think it was, and got a message 2 weeks prior to my exam telling me the site would be closed for renovation, and that I need to reschedule... problem was, my exam was on the last day of testing in september, and the next available test would be in December,
the prometric people were the least helpful people on the planet! they did not help me sort anything out, and I lost two weeks of studying with all the stress, but checked everyday for an opening, and found one a week before my MCAT...
several days before my exam, I found Prometric had opened up seats on an extra day, none of which they told me they would do... and by that point, I'd already found myself a seat...
and seats might still be open a week or even several days before an exam because when the exam approaches, many people reschedule or cancel, and their seat opens up