Personally, I have never used a travel agent for personal travel.
If you are willing to be patient and do your research, you can put together a great vacation... especially with all our help here!
You do need to decide the major issues first. One island or two? What is the purpose of the vacation - lounging or exploring? Budget?
Personally, I would never come to Hawaii without spending time on the Big Island... it is my favorite of all the islands. It has the best of all worlds....