We have found a shared shuttle to be very convenient. As Eljefe indicates, Cabo Transfers is good. We also use Transcabo. http://www.transcabo.com/
They also offer "round trip" to and from the airport, however, due to the negotiated rights of taxis vs shuttles for transferring people to and from the airport, Transcabo will pick you up at the airport, and give you a voucher to use with any taxi company when returning. You then give that voucher to the taxi driver upon return and they then claim that voucher with Transcabo for reimbursement for the amount noted.
We have found that some taxi drivers are not familiar with this concept. So our preference: Take a one-way shared shuttle to your resort. Take a taxi back. The taxi back is only around $26 USD. The taxi from the airport is much more (around $60 USD).
Edited: 12:26 pm, April 08, 2012