They are not awful but do not expect any kind and friendly service plus good luck to you if you have to cancel or make changes. There is a stiff penalty for cancellations. Also getting any due price reductions as they happen is almost impossible, I recently booked a cruise with a trusted agency and paid $50 more p.p. that I would have using C.V.C. just because I knew that if something goes wrong it will be handled with respect and professionalism.
I booked with CVC directly throught the Internet and never had to deal with their customer service, so I can't comment on that. CVC was the cheapest that I found after much research and never had any problems.