We rented a Jeep in Cozumel and had fun cruising around the island. If you're going very far from town, like to the lighthouse at the southern tip of the island or some of the beach bars on the windward side, it's definately the way to go. We had no driving hassles. A Jeep is necessary to get to some spots likethe lighthouse as the road is highly unimproved.
As for San Gervasio...I wasn't knocked out. It looks ruined. There aren't any sizeable structures like there are in Tulum. Plus it's in the middle of the island and it's hotter that blue dammit there.
Your old pal,