I have and do use my GPS on my cell phone, but I don't depend on it because I could loose the signal. I was fishing on my local lake one morning, and while running down the lake I went into a fog bank. I thought it was realy cool how the fog looked like a solid wall as I got to it, I might have been going 30 mph. As soon as I got into the fog my visibility dropped to about 30 feet, so I slowed down and turned right toward the closest bank that i shouldve been near. The lake might have been a half mile wide in that area. After several minutes I couldn't find the bank, and the water depth stayed the same, so I figured I was still in the main channel. I turned on my Google map on my phone to find my location. I was idling in a circle the whole time and didn't know it. That pulled my fat out of the fire that time cause there are lots of fast bass boats on that lake, and have known some people to run hard in fog or dark and rely only on their GPS units. Been several people killed for stupid stuff like that, so I stay near the bank when low visibility.
Don't rely on your cell phone to much. If it looses signal you may be in trouble.