I've found them to be completely sporadic. You'll find some with 2 reviews on a dealer and another with 4, then each of them at a certain dealer would be polar opposites in opinions. The thing is, people don't really go searching for these sites to make actual comments unless they have issues so you'll have to consider that a great deal. On top of that, the good remarks can also come from a dealer's cousin just to get their rating back up - yes, they can be that shady. I've heard of dealers who would sell an ordered vehicle to someone who simply pays more than you a few weeks later, and turn around and tell you there are delays at the plant for your order. You just never know and the reviews are too few to discern anything well enough, unless it's overwhelmingly bad but even good dealers will have unsatisfied customers. Just my opinion.
I also think if you can get the same deal at multiple dealers (making sure of final cost at delivery), then you're pretty much home free. Service can be done at any Chevy dealer later on.