What an awesome experience! My friend and I came here during the "rainy" season in October. While it did definitely pour at times, I did not feel that the rain impeded our good time at all. In fact, most of the weather was gorgeous. One night it rained and thundered and we sat in the observation tower and listened to the storm. Long story short - do not avoid the "green" season - there is plenty to offer.
The farm itself is great. The whole area is full of birds and plants and looks well manicured. We stayed in a bungalow, which was clean and well kept. It was bigger than I thought and very comfortable. There are countless things to do on the farm during the day. We explored the trails and visited the pigs and other animals. We even saw a sloth up in a tree on our free farm tour.
The food was pretty good, nothing too special for the price. I will say, though, I usually have stomach problems and all of these were completely gone while staying at eating the food at the farm. It is fresh and much of it is actually from the farm or at least close by. Try the tumeric dressing. They grow the tumeric on the farm and I think I put that dressing on everything, even pancakes. It's awesome. If you dont want to eat at the farm, you can drive to La Fortuna for dinner, but it is also a little pricey/touristy.
The location is great. Rent a car and explore the hot springs, hanging bridges, Arenal volcano... if you are up to it hike Cerro Chatto. It is super hard (for a New Yorker at least...) but worth it. If you are not a tour person, you can do most of the "tours" they offer on your own by going on the trails or having a car and driving to the nearby attractions. It is nice to have someone pointing out things to you, but if you are on a budget or don't like groups it is good to have the option.
All in all, come here! We spent 4 days at the farm and exploring the nearby areas and it was GREAT GREAT GREAT.