Just a datapoint: I used Globogo, as recomended upthread on post #22 . Their website interface isn't the friendliest (I actually can't find the website now! but e-mail [email protected] should work) but the price was good (526 MXN) and the process was real smooth: I got an instant confirmation by e-mail, and a follow-up contact asking for any further details/requests. Looks like the actual transfer is handled by Posadas group. Driver wasexpecting us, van was spacious, clean, and driver corteous. Highly recommended!

UPDATE: actually the website is slow but works. Pick one of the Cancun hotels on the dropdown menu nearest to where you're going and you can add a note with your actual hotel from there. globoGO Transfers And Tours