Used it for the first time last weekend, I would have to say it was spot on, but it all depends on user input or else it just gives you an estimate. I only found one instance where it was off, but it was only by 10 min, and the previous person who updated it had done so over an hour before I checked it so naturally a wait time is bound to fluctuate between that period.
I did find however,that disney wasnt posting the correct times on the actual attractions, went to ride tot and on the stand by entrance sign itself it had a posted wait time of 30 min, but we literally walked straight on to an elevator with only 2 other people sharing it with us. When we walked out it was still posted at 30 min, very weird they wouldnt update that quicker.
All in all Ride hopper works great when people update it, and its free! thank god i didnt need to spend $10 on the verizon disney app