We've done San Gervasio on our own. Just took a taxi when we got off the ship....and he waited for us, to take us back to the port. The roads getting there are paved and it was a nice ride.
My husband and I have been to one of the largest Mayan ruins, Tikal in Guatemala, but we wanted to give our son a chance to see some ruins. We didn't want to take the boat to the mainland, friends who have gone to Tulum have been disappointed. San Gervasio actually had more to look at than I was expecting, more than what pictures I've seen of Tulum, and the "beach" at Tulum is laughable.
You will definitely want bug spray for the mosquitoes... we didn't, and while it was annoying, it wasn't horrible...but next time, I'd spray. The grounds are well cared for and only two of the paths furthest from the entrance were dirt. No mud for our trip, but I could see that if it had rained recently you'd want to take the advice of the previous poster.
We had plenty of time to wander around town if we had wanted to, but since we've been to Cozumel many times, we went back on the ship and enjoyed the nearly empty pools.
I recommend going, it was interesting, and we had a good time.