The TA will do the leg work for you. They will do the calling or online booking, you shouldn't have to be the one waiting on hold, etc. Make sure if you use a TA, you find one that is a Disney specialist. Be aware, if you book through a TA, Disney will not talk to you about your reservation, because the TA "owns" it until you check in. Example would be if you want to change resorts or a discount comes out, and the TA doesn't gake care of it in a timely manner, there isn't anything you can really do. I don't think it is common for people to have issues like that with good TAs, just something to be aware of.
We haven't used a TA for Disney, but with our schedules, one of us has time to make the calls to Disney, and we don't mind doing our own research. I have seen plenty of threads here of people who were happy with the TA they used.