The bluetooth mini ELM 327 code reader is very small and can sit connected in the diagnostic port without getting knocked by your leg as you drive and it does its thing.
I paid a couple of bucks for the full Torque app on my Galaxy S4 and haven't explored all the features. Its OK but I guess its not as good as something like Opcom for Vauxhalls. Anyway, it least it can provide fault codes if any, and record engine data as you drive.
Cheers, Paul