I have done both. . .and unless you find an authentic Disney expert, then I end up finding better deals. The last time we used an agent, we went through AAA and I had to keep calling them because I found better deals or codes for them to use to book.
The last two trips were short, I had an annual pass, and only needed to book a room, so it was just easier for me to book. Plus, we have direct flights via allegiant and I just book through them.
And, planning is part of the fun for me.
I guess what I am saying. . .if you find a good agent and you have multiple things to book, then it might be easier.