Charcloth: I think if you are considering a local agent, the best thing to do would be to talk to other musicians who are on the agent's list. They would likely be happy to tell you what their experience has been.The national organizations (William Morris, etc.) have a code of ethics and I think they pretty well stick to them. It is very difficult to get with one of those agencies, however, unless you have already been very successful. In other words, get big and they are interested in you (chicken or the egg).
If possible I would suggest you find a local agent that does not have too large of a list of clients. You want them to market you, and if they have too many they might concentrate on what they consider to be "sure winners" and neglect the unknowns.
As Rick points out, a good agent can do a lot for you. They book dates, negotiate fees, take care of the paper work including distributing publicity, etc.
Someone mentioned (English Jon) that he receives the pay for the gig and the agent bills him for his fee. I think that might be a bit unusual. I think in most instances the venue or promoter pays the manager, he takes his cut and the talent is paid by the manager. Things may have changed since I was involved, but that's the way it worked when I was active in the entertainment business. I think if you can find an agent that would agree to the arrangement English Jon has you would have the best of both worlds, assuming the agent did a good job for you.
DougR