We used MouseWait while we were there two weeks ago, March 10-13, including two days that were 8.5/9 for crowd levels. I found it to be very accurate on wait times. The only time it was WAY off (by more than 5 minutes) was when the wait time on the app for Mickey's House in TT was listed at 10 minutes, as was the sign out front, but the actual wait to get into to see Mickey was 35 minutes. It was a real bummer as once you've committed to waiting with a 7 year old, you're pretty stuck. I can only guess that it had been updated by someone walking past, but they didn't go inside.
The only other place where it fell apart was when it would report that no FP were still available, and we would find that they were.
For all of the big attractions, it was pretty spot on.
I would definitely use it again.