My experience is this...
At auction the books do not perform as well as CGC, at BIN I have sold them within 10% (we are talking mostly copper, modern, hot books, variants). On keys, I have not had to discount, although they may not sell as fast. I have found the grading with CBCS to be very consistent ( I have had about 120/130 books graded by them.), but have not submitted in about a year (I took a hiatus). So all in all, I view them to be on par with CGC.
Now.... Not having a registry really hurts them, not having a good message board hurts them as well. CGC has made some stellar decisions there and THAT to me is what separates them, the competitive nature of collectors, the ease of keeping track of your collection, the ease of buying and selling on a message board (that is active). Are clear cut reasons why CGC continues to perform better.