To Chris and Pedro - Yes, not easy and not cheap, and from what I have heard from others, being there to oversee things is not necessarily a guaranty that things will go well. I chose to go with a builder with an excellent reputation (Graniel) and have not been disappointed thus far (we are about half way through construction). Our initial plans were for a simple beach house, but we ended up going with concrete. The water cistern sits below the house and so a good deal of the construction is in the ground. We also have dual septic systems.
The negatives I would have to say are cost and limited choices for tile, appliances, fixtures etc. Benny's is no Home Depot. You can have things sent from the States, but my preference is to just learn to live with what is available locally.
On the plus side, construction can be customized quite easily. We actually took plans from a house in Florida (frame, bungalow, gable roof) and had it redrawn for construction in concrete. We used the original floorplan for the raised level of the house and added another living area (completely different floorplan) and bodega below, had the roof changed to a hip roof and added a verandah in front. Try doing that with your local Florida builder!