Getting to Merida
Convenience: From the U.S., there are direct flights into Mérida's international airport.
The most frequent come from Houston (Continental). (Delta flies to & from Atlanta, but flights out of Atlanta leave at 8 am!).
If the days of those flights don't fit your schedule (or you are using miles and can't get a flight to Merida), consider flying into Cancun.
Economy: Flights to Cancun are plentiful and cheap.
For example, Mexicana flies from Miami to Cancun. American flies to Cancun via Dallas-Ft. Worth.
From Cancun, it's a 4-hour ride to Merida. The Mexican buses are superb - air conditioned, rest room, plush reclining seats - plus two movies en route - all for about US$25-35!
In the past one had to travel 25 minutes to the bus terminal (For only 1 or 2 travelers traveling light, one could take the bus that leaves every half hour; otherwise, a cab had to be negotiated). But now there's a direct bus from the airport. What could be simpler?!?
Of course, you can rent a car, but the drive is flat and boring, and the tolls are very expensive, not to mention gas.