I haven't done it, but I am going to apply soon; I graduate in two months.
I think there a lot of positives. You get a guaranteed salary, you also get 50k tax free for paying back school debt. You'll treat patients that really need the dental work. You build valuable experience. And it's only a two year commitment. And since you are just entering school, you can apply for a scholarship and have all of your tuition paid.
Disadvantages is that you will likely be living in a more rural or semi-rural setting (maybe you might prefer that). You will not likely be doing much complex esthetic stuff.
Hopefully someone who has done it will chime in. I have talked to an upperclassman finishing his first year with the NHSC and he felt it was a totally positive experience.
If you get the scholarship they only send you to clinics that rank 17 or higher on their HPSA score. In Alaska, those are the clinics out in the middle of nowhere (inaccesible by road). In Washington, they are all prisons. Start your search here: http://nhsc.hrsa.gov/scholarship/
Also, the scholarship is extremely competitive. It's much easier to do the loan repayment after you graduate.