Thought I would post this on the chance it might help someone else who's considering the bus from CUN to Merida. I had some before-trip trouble trying to nail down specifics on what I'd encounter after arriving at the airport. And in case anyone doesn't know, you need to get a ticket to go from the airport to downtown Cancun, then another one to go to Merida. All of this can be accomplished at the same time, but you might not know there is no bus that goes directly from the airport to M.
The whole experience went very well, and we'd do it again. We landed at Terminal 3, as I'm guessing most of you will. And that's where you want to be because the bus from the airport to downtown stops there first, then goes to T2. (Exit the terminal and wind your way to the right. It stops at the first of a series of concrete platforms) That means you get first choice of seats, though it's only about a 30-minute trip. (So keep that in mind. During my initial research, someone posted about landing at T2, then walking to T3. If I didn't have any luggage, maybe, but it's not really that big of a hassle to wait there at T2)
Another advantage is that in T3 (no idea about T2), as you're exiting, there are several booths on the left, a bit past the atm. The last booth in the line is for the bus, which means we were able to buy our tickets right there before we left the airport. The clerk had all the times, prices, seat availability, etc. Also offered opinions on which seats to choose. Very, very helpful. (And there was no line at all, but there was a definite one when we got to the downtown terminal. Very glad we already had our tix) Because of where we were staying in M (The Luz) , it didn't matter whether we went to the Fiesta or the main station - it was just a matter of which times and seats were best. We wound up in one of the new ADO Platino (?) buses, which was extremely comfy. It's the most "expensive", but given that not much is really very expensive in Mx and that it would get us to M earlier than the others, we took it. Again, we were glad we did and would do that again too. Quiet, dark, smooth ride...We both were able to get a bit more sleep so that we could get to the city and start the celebration.
Also so you'll know, the downtown Cancun terminal had a nice enough waiting area...free coffee, good restrooms, plenty of seating, etc. Took our bags up to the check-in, got claim checks, then able to relax, walk around, etc. Once arrived in M, an easy 40p taxi ride to our hotel.
To finish this, I'd like to warmly thank those of you whose tips we used. We had a thoroughly wonderful time...loved Merida...never had our pockets picked (to cross threads)...always felt safe walking around...and will definitely go back one day. What fine people, tasty food, great vibe, lovely architecture... My tip to any future traveler? Learn some mayan - Now that was fun!!