I have a dr-680 as well. It is great on days I am double booked, doing sound design, or just don't want to haul 70 extra pounds of misc equipment. The sound is very clean and transparent, pres are quiet, headphone amp is noisy, but much better than the handheld DR 100 is have tucked away somewhere. I think the menu controls are a P.I.T.A. but it has a digital input which is great when you want to travel light but want to input a high end amp/converter combo. You won't find a better multi-channel recorder for anywhere near that price.
The only problem is Tascam build quality is rather plastic-y. I dropped my DR-100 once and a few things shattered (still worked though), I don't thing the 680 would fare much better.