We use it as a general guide for crowds and parks, but as with anything these are best guesses even when based on current data etc... We enjoy using the app for views of hotels, rooms etc. but the main reason we use it is for the FAX function. The ability to use a system that is familiar with the jargon and short hand of the Disney Hotel booking CM is worth the $12 or so for the year. We have had great luck with the system so far and will be using it again this coming trip in August, so I guess that will be a good indication as to its continued use! However, we find it useful to put in our plans and see them in real time and somewhat tangible, this is a great way to iron out bugs or little details you may have missed etc...
We sometimes use the app for wait times and the like, however, as above it is mainly for the room choice.