This is my family's first trip to WDW and we are using an on-line travel agent. One that is based out of Orlando and is a certified Disney specialist. I must say, from our personal experience, we are very happy this is the route we decided to take! We have had no problems getting in touch with our TA through e-mail or phone. All the TA's at this agency have blackberry access and all e-mails have been returned usually within the hour, if not minutes. We like it, because we just told them all of our specifics(room requests, dining requests, package type, etc) and our TA did the rest! On Monday, they had 20 agents calling at 7am to change all eligle reservations over to the free dine(thus saving me time...especially when I have a 3 year old and a 9 month old). At my 180 day mark, the reservation team at this agency had 15 people calling at 7am on a Saturday to make my ADR's and other clients as well. Again, what a great time saver! I have had many changes to make, all were handled efficiently and correctly. Plus, this particular agency has extra little freebies for booking with them like a pair of free mouse ears, small disney plush, postcard from Mickey for the kids, etc.
So, there you have it! For us, it's a time-saver. But, if you prefer do things yourself as some of the other posters have said, then go for it! BTW, we do not get charged for the TA's services. They get a commission from Disney.