The GPS reception on the Iphone is weak. I tried it at laguna and it could never get precise enough to capture the same point twice, hence some laps included multiple circuits. if it had worked then Harry's offers some damn good features. Worthless for me though.
They do sell a GPS mount which is supposed to improve the reception of the phone but it alone costs 130, plus the tom tom app at 50 for a semi decent gps system.