My sister was married in Costa Rica is 2012 and everyone who came had a blast - it was truly magical.
Costa Rica is an incredibly beautiful and romantic location. But it is what is commonly referred to as " hard adventure travel" because it does not yet have the infrastructure to support large scale tourism. This is good in one way because the habitats remain natural and visitors get to experience animals and scenery that is unlike anything anywhere in the US. However, it also means that all aspects of a Destination Wedding or vacation need to be carefully planned. Guests need to be properly prepared for what they will encounter and travel arrangements need to be thoroughly planned in order that guests not require a vehicle. The roads to Arenal pass through some rural areas and mountain towns. Roads are often not paved and frequently washed out with heavy rain. GPS is not reliable even when it appears there is a signal. So as long as everyone knows they are embarking on a true adventure and is up for it, you will have an amazing wedding there.
The seasons there are primarily distinguished by the amount of rainfall. I don't know if your date is set but that is something you will want to consider.
The "Ticos" (Costa Ricans) are wonderful, kind and gracious people.
You can't beat the backdrop of Arenal Volcano - when you can see it. It disappears often behind a shroud of clouds then magically re-appears just as quickly.
So careful planning and a sense of wonder matched by a sense of humor will serve you well as you embark on this adventure!