Even as a gateway it is only mediocre, unless you already live there. The connections from IAD, EWR, ORD and IAH require 3-5 hours of layover in SJU and only can connect to the 6pm SJU-PTY flight. That gets to PTY late enough that onward connections aren't all that great.
If you cannot get to PTY direct from the mainland then SJU is one of a few CM stations worth looking at for the connection, especially as it saves an immigration experience in another country, but the timing isn't great with the connections there.