Publicists can get a lot of heat from authors. Authors often assume that by hiring a publicist, all sorts of guarantees come with them. That said, many good things can happen with the right publicist. As with most things, you should do some research to find the one that's right for you and your goals.
Mark Fortier has been an author publicist for over 15 years. His interview with "Bestseller Interviews" reveals great insight into the author/publicist relationship.
From the interview:
"BI: What is the role of a publicist in making a bestseller?
MF: Publicists aren't alchemists and can't create bestsellers with a magic wand, but they can absolutely maximize a book's media and sales potential. An experienced publicist knows which media outlets have the most sales impact, how to customize a pitch to these outlets for the best chance of a booking, and how to use the right timing and momentum so you have enough sales velocity to climb a weekly or monthly bestseller list. A good publicist acts as an author's promotion partner. He or she should be as driven as you are to reach your goal, and wise enough to steer you in the right direction."