I got my original CR driver's license in 2006, along with the motorcycle license. It was a piece of cake. No appointment necessary. There's several doctors / clinics around the MOPT building that'll give you the "required" exam for about 6,000 colones. I had a local taxi driver run me through the whole process (which helped).
I arrived at the airport and told the driver that I wanted my CR driver's license. I told him that I'd need his services for about 3 or 4 hours. We went to the SL, dropped off my bags and off we went.
Simply bring 3 copies of your current USA driver's license, and 3 copies of your current passport. Get yourself the medical certificate (doctor's exam), pay the applicable fees, they take your picture and 10 minutes later, they give you the license(s). The entire process, from the time I left the SL - to the time I got my license - was about 3 hours.
The original license is good for 2 years. I renewed my CR license in April, 2008. This new license is good for 5 years. The only thing different from getting the original license - versus getting the renewal: I didn't need to show them my license from the USA. Other than that, everything else was the same.
Hope this helps.
MG