A TA can be great since the good ones probably know more about WDW than most of us do, and you wont have to wait on hold forever when calling Disney. They can also help you save money sometimes.
I make all my trip reservations myself, because I "need" control over my reservation. If I use a TA, then any requests or changes I want to make would have to go through the TA -- I would probably drive the poor TA crazy!

But if you aren't "controlling" then a TA can really be an asset in planning a vacation! I just want everything done when I want it done, how I want it done, etc. and dont really trust anyone to be as obsessed about my vacation as I would be!