the on-line bookings are notorious for lack of customer service, great prices, I have done it myself several times.
Lumping all online agencies together is as big a mistake as any other statement that that refers to one persons limited experience with a handful of cruise bookings. There are literally thousands of online agencies. Some are small and some have hundreds of agents and do multiple millions in bookings per year. Each has people who are experienced and people who may be new and in training.
I can tell you for a fact that there are online agencies with great reputations, training, policies, pricing and personalized service second to noone in the industry. Are they all great? No. Are there some you should never even consider doing business with? Yes. The same is true for local agents and there are some agents working for the cruiselines who are as bad as any in the industry. They also have some dedicated people who will go out of their way, above and beyond to make your experience with the booking as enjoyable as possible.
Generalizations and stereotypes are a very bad way to give or get advice. Due to board rules I can't be more specific but believe me, there are great agents in each area of the industry. I've come across many on this site I'd trust with my business and know many at the cruiselines by name I deal with every day who I'd also feel very comfortable calling tomorrow to book me a cruise if I couldn't provide the service for myself. There are also a few I know by name who when they answer I ask to hold and call in on another line. Then tell the first "Nevermind, I didn't need assistance after all" so I don't have to deal with incompetence.