Pretty good for a free app

Seems to be quite accurate in open air when checked against a topo map. It gets the best accuracy (within 10 feet) when you let it sit for a minute at the same location with iPhone back to the sky. In a moving car it went back and forth between 23 meter and 70 meter accuracy, which is acceptable for casual use. There is an occasional bug where it reads way low on the altitude, but restarting the app usually fixed it.

Jwsudd about myAltitude - free altimeter for climbing & hiking, v1.5