The TechHive team used the Compass app in both iOS 6 and iOS 7 on a range of phones from the iPhone 4 to the iPhone 5s and 5c. All phones were re-calibrated several times, and the testers found that the results varied greatly regardless of location (inside or outside). The expensive phones were tested against a reliable and cheap -- US$15 -- Suunto A-10 compass.
Tests with an iPhone 5s showed a variation of 10 to 22 degrees, while a 5c showed more consistent results of 9 to 12 degrees off. The iPhone 4s showed an average of 14.5 degrees variation. TechHive found that the worst deviations came from the last year's iPhone 5, with one phone showing anywhere from a 15- to 28-degree deviation.
By the way, TechHive took on the testing initially as a way of verifying Gizmodo's claim that the iPhone 5s "level sensor" is out of whack. TechHive found that to work just fine, but noticed that the direction-finding capability was horribly wrong.
It should be noted that the test results are associated with the Compass app, not the iPhone's GPS capabilities. However, it's worth considering the purchase of an inexpensive -- and much more accurate -- traditional magnetic compass if you're heading for the deep woods.