I never trusted travel agents before the freedom of the internet gave us access to cost information, and I would need a great excuse to trust them now. Online travel purchasing gives us the freedom to have price information come directly to us, rather than to a middle man who may or may not have our best interests in mind.
So flights I'll buy either through Expedia or from the airline directly (esp if they are having a "sale"). Rooms I almost get through booking.com, my favorite site because of the detailed information they provide on a property and because they've never disappointed me. Car rentals are simply a matter of price, but I often book through Expedia or Priceline.