Off base at Osan is the shit. Do it if you can!
Had a 4 bedroom apartment w/heated floors, bidet, heated parking garage, security system and it rocked. South Korean internet is way fast (shitty on base), so Skype, streaming HD, iTunes...way nice.
Furniture is a bit hard to come by for a reasonable price. Try to get some basics included in your rental. That's not unusual. I had a bed, table, couch, coffee table, and chairs included. A couple hundred bucks at the BX later, you're pretty much G2G.
Bring the family and have a blast. Cabs from Songtan to the local apartment complexes are just a few thousand gonk (< $5).
I'd go back in a heartbeat. My liver on the other hand...