Army: Seem to need bodies right now. From what I have heard there are some retention problems due to long repeated deployments over the past several years. Academic qualifications aren't extremely rigid compared to Navy and USAF. Probably will see time in the sandbox at some point. You do go through some real military training so that you are a soldier as well as a lawyer. JAG school is in Charlottesville, VA (great campus).
Navy: Tough academic requirements to get in. Probably the most connected in the civilian world if you want to use the job to further a civilian career. Practice is highly specialized; a lot of it is getting outsourced to civilian attorneys. Strong departments in environmental, international, and procurement law. Least likely to get shot at, but they do have JAGs helping build local governments in Iraq, so there is always that possiblity to see real combat. If you are serious about the Navy, better put your name in the hat as soon as possible, because there are hardly any spots for experienced attorneys. JAG school is in Newport, RI, where you will have marines and coasties in your class.
USAF: Similar to Navy. You are a lawyer first, airman second. I had a Lt Col who was the staff judge advocate at a large base tell me, "We just practice law here." There are definitely overseas opportunities, including in the middle east, but most likely you will be a government lawyer who wears an AF uniform. They have several programs for where you can contract while in law school. Academic requirements aren't easy but they aren't terribly restrictive either. If you are in the top 30-40 pct of your class from a decent school you should have a shot. JAG school is in Alabama. AF has some great bases (AF Academy, Hawaii), and some really bad ones too (Edwards AFB).