In a week-long trip, I might hop once or twice. Typically, that's to couple a morning in one park that closes early with a later evening in another. If I am staying in the EP resort area, I might hop more often because Epcot is so easy to get to.
The big advantage of hoppers is that it gives you some extra flexibility in getting a "just so" itinerary. That's especially useful when, say, MSEP only runs a few nights each week, and those mornings are much busier in MK. During such seasons, I prefer to spend the morning somewhere else, but see the parade that night. I also find that trips with hopping feel longer because I'm mixing things up more.
The big downside of hoppers is that they tempt you to do more than you really ought to do. It is perfectly possible to have a wonderful vacation without hopping even once.
My advice: do not plan to hop, and do not buy it in advance. If something happens and you need to change things up while you are there, you can always add it then.