Pros
Unlike salaried employees, headhunters generally work on commission. This means that they only get paid if you get hired, and in many cases, they don't actually get compensated until you've been at the job for a certain period of time. The result? If you enlist the help of a headhunter, he or she will be motivated to find you a job, and match you up with companies that are likely to hire you.
Cons
Some headhunters will try to get you in the door for as many interviews as possible in the hopes that an offer will come through. But that could make for a very demanding and draining job search. It's often the case that less is more when it comes to interviews -- you're better off meeting with a few good companies than wasting your time discussing roles that aren't really right for you.