I'm using this nice little device now since half a year and i'm really pleased with the quality. I did also a lot of research and i found a comparision between the sd preamps and the ones from the dr680 and there seems not to be the big difference. I mainly use it with rode mics (ntg3 and nt4) for sound fx and ambience Recording. I run it with an external battery adapter with sony camcorder batteries and they work a full day without any issues. The only thing, what is disturbing is the metering section of this device. You can't get propper visualisations because there are No numbers and only a -12 db Line, which is shown. I miss a better metering visualisation. If that would be done, i think there wouldn't be a reason to switch to a sd recorder or something similar.
Go and get it. I don't thinks, there's something better on the market fot that price.
Check out my soundblog with a lots of recordings made with the tascam dr680 Www.klang-manufaktur.de/aktuelles
Also check out my sound fx library at www.thesoundcatcher.com It was all recorded with the Tascam dr680