I've tried several running apps on my iphone and the GPS fails on me, I've been toying with the idea of getting a garmin and my hand twitched over an eBay listing... I've now got a garmin!!
I ran yesterday with both monitoring, once again my IPhone could not find GPS, Nike + told me that I ran 4.92 miles the Garmin said 4.20 my parter asked me "whats the difference?" the difference to the average pace per mile is 9.46 To 11.16! I'm much slower than I thought! Oh well!