We did this excursion on the 11/8/2003 cruise. The ferry ride was a bit rough, but we all did fine... though people around us were sick. The staff was handing out little pink bags to many, many people. My DH who gets quite seasick had taken Bovine and had a SeaBand on - and sat by the window so he could keep is eye on the horizon, and actually felt better at the end of the trip than I did! (No band, no bovine.) There are various seating options - and even a t-shirt shop ($5 shirts) and a bar. The ferry has comfortable seating, air conditioning (lower floor) and tv/movies to watch.
There were no bugs at all at Tulum when we went. We had plenty of time to wander around the ruins alone AND enjoy some beach time. Lots of people were having a blast at the beach, and especially the kids... though the surf was rough on that particular day. The bus ride passes quickly - on the return trip they do not stop for a bathroom break...they do it on the way in because they want you to shop.
There are bathrooms at Tulum that cost .25 and are attended and clean...unlike the bathrooms at the potty stop on the way in.
For us, we enjoyed this excursion simply because we were able to see a place we all REALLY wanted to see. My son was disappointed to discover you can no longer go in any of the buildings or climb the main tower. He had a book about ancient places that he had since he was five, and at that time you could enter the buildings. Our guide explained they recently did away with this because people were defacing the ruins.... that was a disappointment for our son initially, but he regrouped and we were just amazed at the beauty of the location. It really is amazing if you like history. We got him back on track by thinking about all the footsteps that went before him...and how it was like to live there back when it was a thriving city.
Overall - we enjoyed this excursion and would not have planned it any differently. If we go on the Western again, we will skip this, but simply because we've already been there and felt we saw and did so much, we do not need to visit it again when there are still so many things to see and do in and near Cozumel.