Wife's car was shuddering and had the check engine light on.
Figured that it was a failed ignition coil, as symptoms were the same as for a similar problem last month (which an Indy charged €120 to diagnose and replace).
Anyway, I downloaded a free OBD App for Android and picked up a Bluetooth ELM327 OBD dongle today (€20 from a bloke on Adverts.ie)
Problem diagnosed in 20 seconds from my phone!
Fault: Ignition Coil #2 misfire.
Spot on - replaced the coil (€31 from Micksgarage) - problem was solved.
Used the same app to clear the fault code and turn off the light.
Very impressed, and works on nearly every car with OBDII.