I own and play with many of them.
The APP I recommend is the AB Mobile App. It costs around 29$ to purchase app, but it is the same program resident to the Kestrel 5700 with AB.
The biggest advantage to the AB app (Applied Ballistics is the company owned by Brian Litz who is literally a former Rocket Scuentist and the chief ballistitian for Berger Bullets) is the ability to purchase Custom curves for your specific bullet for $1.99. Manufacturers will publish BC's for their bullets.
G1 - Think flat tail Spitzer
G7 - Think modern Boat Tailed Bullet (What want to use if not using custom curve)
Brian takes the bullets fires them in test rifles using Doppler Radar and from the derived empirical data calculates custom curves representing how the bullets perform in the field. Admittedly the bullet May perform slightly different in your rifle which would lead you to further tweak the ballistic curve out near the transonic regime to give you your own custom curve for the bullet in your rifle (something you can do in the Kestrel 5700). But, in my experience with all of my rifles the custom curves have been spot on to transonic and beyond.
In short for 29$ and a free barometer App (Station pressure) you can turn your phone into a kestrel minus the wind meter. (Think a Kestrel 5700 with Link sells for north of 600 just as an FYI)