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.