i have an agent in australia which i use for pretty much anything that isnt related to music,film, fashion pop culture etc - they handle all forms of advertising work that i do as well as some other stuff that i just dont feel confident in quoting. It works well for me as our relationship has always been one where they dont touch anything that i have worked my ass of to establish but have been fantastic in finding new work outside of theses realms and in particular look after my copyright and terms of use etc. I would never look for an agent to look after everything for you though. I think it will always work better if you still create your own work and use them as a help rather than something to rely on. Anyway - my two cents
