Boy this is a flammable subject.  The reason Dexicool seems to get a
bad wrap is that the same time that it came out, GM had started using
a LOT more aluminum in engines and in the presence of water, aluminum
and castiron can be very reactive.  Some will say otherwise but 50/50
anything does not last long in a engine like this. I discovered long
ago than by using 60 to 70% antifreeze that the effect of that amount
greatly extends effectivity of coolant and its life. THis is for two
reasons, the first being that there is less water agent present and
the second it the higher percentage of rust inhibitors in a stronger
mix.  I hve two vehicals with Dexicool that has been in them for over
5 years and the coolant is still clean and overflow tanks have no
stains in them but they are both between 60 and 70 percent Dexi and
the rest water. I have a 89 burb that has not been flushed since 97
and the coolant and tank is clean and it has about 70% concentration
of antifreeze in it. I have a 51 year old John Deere Tractor that has
not had a coolant change for over 19 years and it see 30 to 50 hrs of
run time a year too bushhogging and such.  It has nearly pure anti
freeze in it (between 80 and 90%) and it is still fresh looking and it
does not overheat on even the hottest day when working hard and it
does not even have a water puimp either (thermo siphon cooling) so the
theory on it running cooler with water has never been seen by me.

I had the same crud buildup, but my car was over 100k miles.  I'm not
sure if it's common, but I drained and refilled the coolant system
myself, and it did build up over time.

As for harmful, well the layer of crud narrows the channels, and
forces heat to have to pass through another layer of material before
being carried off by the coolant system.  Yeah, as a layperson I
figure this is bad.

hth,

tom @ www.CarFleaMarket.com