I tried both G-Tac Pro and LogBox last weekend, along side my Traqmate. G-Tac is my clear preference of the two iPhone apps... it's simple to use, can replay the session on the phone, the data export is two simple CSV sections, and it doesn't let the phone lock when you have an unexpected 20 minute delay in grid. Unfortunately, both suffer from the iPhone's GPS reception issues. It seems that the center tunnel is a great place to mount an accelerometer, but not so great for a GPS antenna. Because of that, I was only able to compare accelerometer data this time. I hope to try this on the track again next month and will hopefully be able to collect GPS data then...
I've had people on multiple forums asking about this, so I put my G-Tac vs Traqmate comparison and video out there as HTML that I can link everyone to...
http://www.rm944.com/weston/iPhone/G-Tac%20Pro%20vs%20Traqmate%20-%20090419.html
As for LogBox, it lacked sufficient GPS data as well (I mounted the iPhone in the same place for both apps), and it doesn't give me a clear time-stamp on the accelerometer samples, so I don't have anything put together for it right now. Basically, it was writing multiple samples with the exact same time values, and it didn't log at a steady 20Hz like I set it to, so that really leaves something to be desired. I would hope that future updates correct that...