This will only be my third Christmas/New Year time period that I've gone to Costa Rica, from early evening Christmas Eve and Christmas day things are rather slow, the same for New Years Eve and New Years day, but other than that, you shouldn't have any problems, if that brief history is a guide. The week before Christmas seems to be great, with the chicas trying to make as much extra money as they can and willing to play let's make a deal. The week after New Years I've heard is also really good, the chica's having to pay for all those presents they bought using rent money.

The main downside is you could be in a sausage fest on New Years Eve with so many guys in town, which is why I am leaving on the 31st.
ShyOne