We have been there many times since 2004. We lived there in 2007 - 2008. We own property in Costa Rica. I don't know the reason for your question so my answers may not be what you're looking for.
I have a hub on here called http://hubpages.com/travel/A-Small-Guide-to-Visiti... . I just updated it a couple of weeks ago. On line information and even printed information that you would normally trust, e.g. Fodor's, National Geographic, etc. can not be relied on to give you accurate information about Costa Rica. That's because it's changing so rapidly. If you read a blog by someone about the condition of the roads, for example in Guanacaste in 2007 they would have said that they were awful. That would have been correct. The 2006 rainy season was really rough on the roads and by early 2007 the previously paved road from Liberia to Tamarindo was a rough, rocky mess as they repaired it. Before that and since that it's been just fine. Primary infrastructure doesn't change much in the U.S. or in most countries but there the pace of change means that 6 month old information is not very useful. That's true whether you're reading about a hotel, a restaurant, even local customs to some extent.
The people are very helpful and friendly. There is no reason not to have a great time there. The variety of recreational opportunities is endless, restaurants have gotten quite good, and much fancier, better hotels have opened though that may not be your preference.
Things you may find about low cost aren't so true anymore either. Not that it's expensive as tropical vacations go but it isn't cheap anymore, probably similar to most of the U.S.
As to weather, avoid September and October. As to location, the Northwest Pacific coast is the best place to visit unless you're looking for a real rugged outdoors experience in which case the southwest area, the Osa Penninsula would be your spot.
Read my hub on here. It will help a lot if you're thinking about going there.