Yes a very handy instrument. As well as all the obvious benefits of knowing what caused the CEL, if you get a pesky CEL simply by leaving the gas cap off by accident, with a code reader one can easily clear the CEL instead of having to wait so many cycles for it to clear itself.
I've heard of stealerships charging $100 for this. If you happen to be unlucky to get nailed with such a charge, the unit you buy pays for itself right there.
A good investment for sure.
Yeah, the days of tuning, timing and plug changes are almost gone
C.