Glow app fertility accuracy?

Hi all! I was just wondering if those of you who are using the app for fertility tracking have had success. 
The app changes my anticipated period due date and fertile window if I add positive or negative OPK results. This month, it changed the date my period was due by 5 days after I had two LH surges 3 and 4 days before my original due date for my period was scheduled. 
Basically, I have no idea how accurate this is and I don't even know if I can trust it to give me actual fertile days or not fertile days! Anyone have any success or negative stories ??