I use this app all the time and even have my 77-year-old mother-in-law checking out the ISS because of it. It works flawlessly and has great functionality.
I recently had a problem using the app after I got a new iPhone 6s. So I emailed the developer Martin who got back to me within a day and helped me correct the issue. Turns out it was an issue with the iPhone compass calibration, not the app. Really great responsiveness!
Jupiter122 about ISS Spotter, v1.5.1