We really enjoyed our stay at the Falls Resort. The property is like a cozy little retreat that is tucked into a pocket of the same rainforest as Manuel Antonio National Park. While there, we saw a couple white-faced monkeys, an agouti walked right past our patio, a Jesus Christ lizard hangs out on the walkway, and the poison dart frogs will probably keep you up at night. (They're really loud.)
I feel that "resort" isn't quite the right word for the place--it's more of a boutique hotel, really.
The bed was the most comfortable of our trip, but the room is on the small side and fairly dark. There's really no where to put your suitcase, But you do get a fridge and a nice TV. It's not luxurious, but it is modernly appointed. The pool area is nothing to speak of--if that's important to you, you'll probably be a little disappointed.
The restaurant was probably our favorite thing about the hotel. Louis took such good care of us, and by the end of our stay it was like saying good-bye to a friend. The menu is very strong, with lots of great choices, and everything we ate was excellent.
We also stayed at Arenal Manoa in La Fortuna and Rio Celeste Hideaway near Tenorio, and while the staff at the Falls was friendly, it didn't compare to the level of service at either of the other hotels. (I get the impression that the Manuel Antonio area overall has reached its critical mass in terms of tourism, and that service suffers a bit as a result. Understandably so.)
If I came back to Manuel Antonio, I might spend a little more and try somewhere else--somewhere with a bigger pool and a nicer hot tub--but I would definitely return to dine at the restaurant.