Great question freecampingaussie. My preference is to book domestic air myself (I usually just use the Qantas website to book). For international I almost exclusively use a travel agent unless I am using frequent flyer points. The good thing about agents is if they get it wrong they pay, not you. Also they know all of the tips and traps about booking in areas that we fly less frequently from.
If you are a member of a motoring club (eg AAA, AA, NRMA, RAC, etc) they normally have a travel agent area that give discounts to members as well as special deals. Worth checking out to save money and to get great advice and service. Cheers Michael