I had a fantastic experience in Costa Rica. I researched on WhatClinic, TripAdvisor discussions, and various blogs where people talked about their experiences, then contacted a couple places to get a feel for what they were like. I ended up going with one of the most expensive ones there, but it was still a better deal than what my little sister paid for her root canals and crowns after insurance here in the US (she has great benefits as a teacher) - and of course, there was no annual benefit cap to contend with.
I'd definitely recommend going during their rainy season, since hotels and flights are cheaper and you'll spend a ton of time at their office and in your hotel room anyway. If your work won't be too painful and you can afford it, plan the trip to be longer than you actually need so you can build in a little more fun time. Most of the better places have coordinators who are happy to help you figure out where to stay and how best to schedule the trip.
You can send your xrays down ahead of time to get a rough estimate of what's needed, but they'll do a new evaluation when you get there. In my case, they told me that some of what my US-based dentists had recommended was completely unnecessary (and that the treatment plans had likely been more about making money). I needed a few root canals and crowns, but not as many as was recommended here in the US. Similarly, the CR dentist said there was no reason I couldn't keep my 2 remaining wisdom teeth.
I'd recommend staying really, really close to the office you choose. Traffic is pretty awful and the driving down there can be a little scary to those of us who are used to orderly driving. I stayed at the Radisson and went to Meza Dental and it was no more than 5 minutes back and forth, even during rush hour. When your face is numb and you're miserable after a long day at the dentist, that's really nice.
The only thing I think I would have done differently is to pack more of my own food, as hotel food started to get really old and food there can be surprisingly expensive, even at the local Walmart. The hotel employees repeatedly advised against going out after dark (about 5:30-6pm in June) without a large group or local guide, so I ate a lot of room service (which, weirdly enough, was the same price as walking to the hotel restaurants to eat).