Straight advice on DR 680 (vs Mix-pre6/10): I have not used the latter decks, but have been with folks using them.
The 680 does everything we need to do 4-6 channel stuff. We typically record PA amplified music.
The pros are well documented: 6 channels, runs well on internals and many external battery choices, menu is pretty easy, LCD screen is large enough to see in the field, will use SD cards up to 64 GB, not huge footprint in bag.
Cons for me:
1] The stock Pre-amps just are not THAT good, they are acceptable, but we have found using outboard pre-amps such as Grace V2 or Grace V3 or SD Mix-Pre going mic in via XLR to the 680 to very audibly improve the sound of each channel. (of course)
2] Phantom powering certain mics just sucks (specifically in our case AKG C414 ULS); however, we have had odd issues trying to run 6 phantom channels. (thumping, no signal(s) etc)
3] Using certain external power solutions seems to be problematic- as in the deck has powered itself off a few times without any "reason".
Fir the price, I feel it is a great solution, but If I am accurately representing his opinion-kindms would not buy one, nor recommend I buy another one, after using this one with me for almost 2 years (for instance)
My final opinion/advice, if you choose the 680, would be to purchase the OADE mod version of the DR-680, a bit more $$, but more versatile depending on if you need/want external pre-amps in the chain.