Lots of comments here on DEF and some good info as well. I'll be the "odd man in" after using a jug of SuperTech DEF from (of course) Wal-Mart a few years ago. Everything seemed OK for a while after adding it (which brought my gauge reading up to ~3/4), then I started getting DEF system error messages. Took the truck to the dealership (under warranty) and they found a cruddy DEF injector. Rather than replace it for ~$300 plus labor, the tech cleaned and polished it to new condition and re-installed it. I have experienced no real problems with it since then, but immediately switched back to the Peak BlueDEF.
Moving ahead to this summer and thirty thousand miles later, I had a DEF error appear while towing. Not knowing when the last regeneration had taken place, I stopped in to a local shop, had the code cleared, and went on my way. I figured if the error happened again, I would be close enough to home and the dealership anyway. The error message never came back and the truck continued to run as it should. Per the advice of the dealership service advisor, I allow the level to reach ~1/4 full before adding a jug of DEF.