The guy who I bought my E30 from over a year and a half ago just sent me a Dinan chip he had for the M42. Of course I rushed out and put the thing in. The biggest difference that I noticed is the throttle response is so much smoother. No more "on/off" feeling when you lift or nail the throttle.
Seems to have a bit more mid range oomph...but certainly doesn't transform the car into an M3. I haven't tested the red line - I've heard this chip is fairly conservative so I imagine the new limit is 6800-6900 or so.
But hey...it was free and it seems well designed.