FYI I've been using a PLX Kiwi for months now. I'm an Android rather than iPhone user. I use
Torque as my default OBD-II gauge application and fault code scanner, and I've also used it with track datalogging apps (e.g.
aLapRecorder HD). Works great for both.
I tried a cheap knockoff adapter first (
this one), and while it "mostly worked most of the time", it wasn't really reliable for datalogging/gauges like the slightly more expensive PLX Kiwi is. Ended up giving the cheapy adapter to a friend who just wanted it to read fault codes occasionally.
Anyways, summary is: buy a PLX Kiwi if you have a smartphone and want better data on your car