Thanks for all the replies!!
I think I'll use the current sense function to detect motor stall or motor overload.
At the moment I have two magenets/wheel. I can increase this to four? It will give a more accurate reading at low RPM, but I don't know if the interrupt can handle (12.000 pulses/minute max.)?
PID is starting to make more sense to me Maybe I'll try to incorporate specific PID settings is certain ranges (like CodingBadly proposed).
EDIT: Maybe I can better use:
http://arduino.cc/playground/Code/PIDLibrary
and
http://arduino.cc/playground/Code/PIDAutotuneLibrary
I am still working on the library... I can post a preview version (not fully tested) if people are intrested?
Cheers.