Great for DE track days
A friend at the track was using it, so I bought it on the spot and used it all day. A little tricky to setup a track the first time (if its not in the database). However, once the stop/start position was set and some interim "sector" points made, the app just worked great. Its really nice to be able to start it in the pits and stop it when lapping ends. I cant comment on accuracy, but the speeds shown are less than indicated on my speedo by 3-5 MPH, but its a BMW which are infamous for exagerating speed.
I havent seen any information on if the iPhone 4 has better or more accurate GPS, but LapTimer is working on mine. My friend at the track was able to send me his Portland International Raceway map from his iPhone 3GS, but the waypoints were all bunched together in the wrong spot on my phone. Easily fixed by dragging them to the proper positions.
As with any app that uses GPS and sensors, the battery will go fast. Use a power source in the car.
Bottom line for me, this app is highly recommended for recording your lap times for HPDE days.
Zenbob1 about
Harry's LapTimer Rookie