I used it all this summer on my HTC. It uses a lot of battery power within a few hours. You must use on boad power supply for extensive usage. Comparing it to my on board GPS and other GPS', the speed is off by 2 to 5 tenths of a mile. The distance measurement is OK, given the small screen size. The real problem is actual GPS position, I have it set for one second update, but from time to time the boat's actual position is about 200 feet from the display on the small screen.
it has it's limits, I trust it 25 to 50 miles offshore. in harbor areas, I keep a very close eye on it