Pros
The best part about the NPS is the people. I have met an immense amount of great individuals. For the most part, they are old, so they have a lot of wisdom to give. Also, they will retire soon which makes it easier for you to get a job. JOB SECURITY! In these tough times, if you're in with the NPS, you should feel pretty good about your job stability. You meet so many different types of people! Expect interesting conversations! You learn a lot. The topic of your site will become a part of you. It pays to be informed.
Cons
Not a lot of growth potential. A range of management. Since growth is usually dependent on movement, it is hard to know what you are getting into when you join up. Some sites are extremely lax, while others are extremely strict. BEWARE. If you don't like people, don't join the park service. You have to be able to network. Without networking, you won't go anywhere in the NPS. Can have a horrible work schedule. Be prepared to work weekends and federal holidays if you work in the field. All the office employees will get off before you. Prepare yourself now.