I used a TA (online) for my last cruise, and it was a NIGHTMARE! It started fine, but when it came time for my final payment, they suddenly wouldn't accept my payment (tried 3 different debit cards). After going through menu upon menu on the phone (this company primarily does airfare, and half the CSR I spoke to didn't even know the company SOLD cruises), someone was able to take my payment.
For this cruise, I'm just doing it myself directly through Carnival. I'd much prefer to deal with them directly, and already have OBC from putting a down payment on my next cruise on my last one.