I am an over-planner and like to manage our own vacation. That said, we used a
travel agent. You don't pay extra for a travel agent (or at least we didn't). The price they gave us was exactly the same as what Disney was offering. I believe agents get a commission from Disney, so there is no extra cost to you.
When we used a travel agent, we were able to secure our hotel accommodations knowing that if a better deal came along that resulted in a bigger discount for us, the travel agent would be able to get those savings for us. If we planned it ourselves, I'm not sure how easy it is (or hard) to change from one discount to another. Additionally, I didn't even need to see if there were new deal. Our travel agent is always aware of the latest discounts that Disney offers and automatically applies those discounts to see if it is a better deal for us. I didn't feel like I had to "wait" to book in case there was a better discount later (like free dining).
As far as making ADRs, FastPasses, etc. I think you can really use the travel agent as much or as little as you'd like. When we were 180 days out, I knew ahead of time which dates and times I wanted certain ADRs. Instead of staying up late or waking up early (I forget the exact time you can book), I just let my travel agent know and she did it for me. We got everything we wanted. Since then, I have logged into My Disney Experience online and adjusted my ADRs, cancelled some, added others. When it was time to book FPs, I did all those myself online.
