I have ordered from all three but mostly from CI, but I prefere Ames all the way. I have delt with CI customer service and they are very good to deal with. I have had a few problems with a couple orders not being right like my oil filter adapter for my long branch manifolds. That caused me some headacks since it was for a full sized car. The rubber hood bumpers were not complete since they are camaro pieces.
I called customer service to talk to them about my front springs and they said they would take them back even though they are 4 years old. So they are nice to deal with but at the same time it can be a pain in the @ss to have to re-ship.
Ames says it best in their catalog that some products have more then one vendor and they figure out wich one is the best/most original and thats all they sell. I very much like the fact that they rate the quality of many of their products. In the beginning I was only buying from CI to get that discount and free shipping but like it was mentioned above it depends on what your getting.
You may want to read the fine print on those 20% off cards they don't apply to kits and some items say there is no further discount. My last order didn't have much of anything that the 20% applied to.