I tend to only research the area, the hotels, things to do locally etc for the place we are going before I actually decide where we are going.
I also ask alot of questions in here. eg: Cuba is our latest destination next year.
Once booked, I usually buy a tourist book or something to have a read.
I like the excitement of actually exploring and not knowing what I might find.
I would research the practical stuff like hiring a car, dive schools etc..the important things to me.
If it is anywhere I have been before, I tend not to bother these days although I may check out the hotel reviews or something to see if anything has changed.