You don't want to go to PV during New Years.
Sure, it is a festive time and lots of bar hopping if that is your thing, but in reality PV is completely overrun during the high season. I was there last December during New Years week and it was crazy crowded. Lines at the airport (waited for HOURS for a cab), lines for breakfast (get up early or forget about it), lines for dinner, etc. And not cheap either. Food prices were almost at the level of mainstream US (not surprising considering the crowds). I agree with London_traveller that the quality of the food was "meh". The weather was a mixed bag. Had a couple of days of rain, some days of sun. What also should be said is that PV seems to attract more of a "LA Party Crowd". Trying to sleep quietly at night is neigh impossible.
In the end, I cut my 7 day trip short to 5 days as I was starting to get unhappy about everything. Was glad when I got home.

I am planing to check out Costa Rica this coming winter. Curious about it as I have never been. Perhaps the beach in Manuel Antonio is a bit more relaxed then in PV.