So today I made a trip to the Tallahassee National Weather Service Office to compare my iPhone 6 plus and Kestrel 4500
barometers to the Precision Digital Barometer on site.  In the above photo I am comparing "sensor pressure" ie pressure
of the sensor only (not corrected to sea level).  If we round off to 1/10 of a millibar here is what we get

1014.3 NWSO Tallahasse
1014.2 iPhone 6 plus
1014.2 Kestrel 4500 BT

All within 0.1 mbar which is pretty good.  The iPhone 6 barometer's sensor is the new BMP 280 made by Bosch according to Apple.
It has an operation range of 300 to 1100 mbars.  The app I used is called Barometer ++.  It determines altitude by GPS and correctly found my home altitude of 14 feet.  I've check it daily for a week and It is within 0.1 mbar of my Kestrel.  At the Tallahasseee NWSO it had trouble with the altitude because I was inside and couldn't get a GPS fix.  Outside for what I do - measure the pressure inside the eye of a typhoon or hurricane it would work just fine I believe.  Look for more iPhone 6 apps to come out down the road that have data logging and graphics.  My next photo is of the station barometer vs my Kestrel 4500 with the correct altitude put in  

 So the difference when you account for altitude - in this case 173 feet - is 0.3 milli bar.  Not bad at all.  The Kestrel is calibrated at the factory to a known standard and in the field you would input the altitude offset.  With an accurate barometer in the iPhone 6 it will be a useful tool for getting a cross section of pressures in hurricanes and typhoons.  The acid test would be to take all three barometers in a pressure chamber and compare values from say 900 mbar and up every 20mbar then you could plot a calibration curve and compare.

11-17-2014 update:  I've found a better barometer called "barometer for iPhone 6" on the app store.  You can manually input the altitude or use the automatically derived GPS value.  It looks more like a traditional barometer and has a set button to tag the current reading - just like a regular old time barometer.  The numbers don't jump around either if you use a known altitude setting. Only goes down to 960 mbar but I'll bet the digital display goes down lower.
    

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