Our 2nd best month (as a group) on record came the month directly after cancelling and the best ever month came in Aug. Hard to say specifically what caused what as there were countless changes being made across the group. Personal is and will always be the number one way to improve the volume in a dealership. We have seen 40-60% MOM turnarounds from GM/SM changes. Website traffic increased 20-25% MOM right away and ended up being 65% YOY by Aug (Jan-Aug '11 vs, '12). This wasn't just from PPC but a spectrum of digital offerings.
A few of our stores are on Autotrader as of a month or so ago but on the whole most are not. Our focus over the last year was to instill the correct pricing procedures which is still not perfect but definitely much improved. There was a time when managers didn't even know how to price a car. The theory was what is the point to advertise anywhere if you are going to be 2-3k off the market.
Our comparison had to do with paying nearly twice as much as a group (AT vs. Cars.com) and I think everyone would agree that at some price point any product, no matter how good it is becomes too much (especially if used poorly as in our case). Our market was dominated by other groups spending 2-3x more than us on AT and close to zero on PPC. So the choice was try a different direction that is relatively soft or swim upstream.
I would say to those asking themselves the question should I cancel, lower etc. you have to realistically grade your current performance in your vehicle acquisition, photo process, pricing and re-pricing process. If these are poor then you can't blame the vendor for your issues. The individuals that support or not support any vendor 100% no questions asked shouldn't sway you from making a decision based on your situation. If someone would use one vendor no matter what in any market or situation then I wouldn't listen to them as obviously they aren't thinking clearly or have another agenda...
Be creative, make a decision and get everyone behind it!