We book all our cruises through Costco - Royal Caribbean and DCL. We've never had to leave a message and wait to hear back because it's a big company. As long as its during their business hours (which are long!), we've been able to talk to someone. They've been a big help in some cases, where Royal screwed some things up. They knew what to say and who to talk to in order to get some results with our issues.
You don't have to do everything through them. We've always been able to book shore excursions ourselves (either online or by calling Royal).
The onboard credit is very nice, and the reason we started booking through them. (Our first few cruises we booked online.... Didn't even know about Costco Travel!)
Costco will sometimes have "member cruises" where there are additional incentives, like specialty dining vouchers, etc. We booked our most recent cruise a year in advance, and about 6 months later noticed it was a member cruise. Called Costco, and they added the package to our booking. Ended up with additional onboard credit, soda package, specialty dining voucher, spa voucher, amenities on Labadee, and I think some other things I'm forgetting. It was pretty neat!
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