1) Learning my original life-plan for after high school wasn't a good fit for me (while getting paid to make that realization).
2) Getting college paid for (with some scholarships I was able to stretch the GI bill into nearly 5 years and got a Math/CS double major.
3) This is the big one: I live in San Diego (a "military friendly town") and having military experience won't guarantee you a job, but many employers here will at least talk to you and give you a chance to make an impression beyond your resume.
A big negative (besides the possibility of risk to life/limb/mental health) is that you give up the "traditional" college experience. It may take some extra time to find where you fit in when/if you do decide to go back to school. However, that's probably true for anybody who delays college for more than a year or two.
My contribution to the discussion would be The Bear Snores On. Some great phrases in there that keep it interesting enough that I don't mind reading it 3 times a day, yet fun enough the little guy still enjoys it.