Very accurate. – Barometer++ : Simple and Accurate Barometer with Widget for iPhone 6, 6 Plus, and iPad Air 2 Review

The app does exactly what it claims to do. The only feature I wish it had graphing. Otherwise, it is very accurate.
Review by rxxdoc on Barometer++ : Simple and Accurate Barometer with Widget for iPhone 6, 6 Plus, and iPad Air 2.