A couple of things worth noting. While I am not a JAG, I am a former Army Company Commander and worked with the JAG office all of the time.
1) If you are not competitive for some of the BigLaw salaries, the JAG may offer more money than other jobs.
a) you get a tax free housing salary based on where you live. As a married Captain in Virginia, my last duty station, the housing allowance was $1887 for 2009. That's over $22,000 annual, tax free.
2) You get a ton of experience. The JAGs I worked with, were trying cases in court rooms, and after 3 years will have a ton of experience other laterals may not have. Not to mention, the military thing looks great on the resume.
3) You experience a lot of different aspects of the law. If you are not sure what you want to do for your career, this may help as well. I sent soldiers to JAG for issues raning from criminal, family, tenant, contract, etc.
Obviously, there are cons
1) For some people this is tough, but you need to uphold yourself to high standard (good fitness, clean cut, no drugs, no DWI, etc.)
2) Deployments - if you are married, they can wear you down. If you are single and don't have to deal with the family aspect, not as much. Either way, deploying is one of the most rewarding experiences of my life. And again, looks good on the resume.
3) Pay is not as good as BigLaw, but the hours aren't as long, unless you are deployed.
Hope this helps with anyone considering JAG. If I had to choose, I'd say go Air Force. They seem to live the most comfortable lifestyle.