I think it kind of depends on what you're looking for. I'm a publicist for a small press, which is completely different from the job of a freelance publicist who works with big name authors or books published by big houses.
Would you want him/her to book a tour? Do you have a day job? Do you just want print coverage? Would you want to be on radio? TV?
If you're willing, I suggest doing as much of it on your own as possible (unless you have lots of disposable cash). If there is something you really want, like an NPR interview, and you just can't seem to book it yourself, you could hire a publicist to do just that task.
I don't know a ton about freelancers or agencies, but I do wish my press had the cash to hire one for a couple of our books/authors. Seasoned publicists have already established and nurtured crucial relationships.
I remember one publicist at a conference talking about her methods. If one of her authors decided to go away for a weekend to Detroit, she'd pull out her contact sheet for that city and start making phone calls. She'd book them at bookstores, radio shows, and TV shows in the area. Very impressive.
I suppose finding a publicist is similar to finding a publisher. There has to be a good fit.