I usually just book online when I see a price I am OK with. That's what I did for our upcoming Panama Canal cruise. I like having control of my own booking and I actually enjoy the details like figuring out where on the ship I want to be, etc. I really enjoy the research portion and always have my own plan of what I'd like to do.
However, I do use a friend who is a TA every now and then — if I am trying to plan a large family vacation. For example, right now we are trying to book a trip to Alaska that my parents are paying for. It's a cruise tour, so has both land and sea sections. We'll have 10 people total, need multiple cabins, etc. When there are so many moving pieces, it is so nice to turn it over to her and have her take care of it. I usually have some basic info of "must haves" like ship, deck, cabin type, dates, etc. and then she is awesome about taking care of the details and making sure we are all close together, etc.