I agree that word of mouth and referrals is the best way to pick a TA. I used to book all my own via a Carnival PVP. The PVP was great but never seemed to be 100% on my side. Meaning that if there was a price reduction I had to find it, if there was a better choice of cabins, then I would have to suggest it. I am sure with the volume of bookings they handle its not practical to give the service level one would expect from a personal TA. I'm not saying NOT to book directly with a cruise line, as it does work for people. But I changed over to a TA recommended here and have not looked back. They offer the same prices as the cruise line AND I feel they are looking out for me.
I would avoid the big "discount" booking companies. They may offer a great deal but if something arises, once they have that booking, they have no reason to be helpful after that.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007