I moved from DR-680 to DR-680mkII to F8 (with some trial use of R-88 prior to the F8).
I'm very happy with the preamps in the F8, which are better, probably most apparent when needing lots of gain for dynamic mics or recording in especially quiet conditions. However if I didn't need 8 channels all the time I would have been happy sticking with the 680mkII for concert recording. It's a great recorder and one of the best values around.
For concert recording using condenser mics I was fully satisfied with the Tascam preamps. I occasionally used a V3 in front, most often whenever I needed 2 additional channels. Yes for some things I'd run the V3 on a main pair simply because its a better preamp, but mostly just because I could, rather than because I felt I needed to.
I do like the improvements of the 680MkII over the MKI. Supposedly the preamps were improved but it's the menu navigation improvements primarily, and a total lack of any problems in hot outdoor conditions that drive that preference. Second hand 680 MKIIs can be had quite inexpensively, and represent a value that is very hard to beat.
I've fully shifted to using the F8 and now keep the 680MKII as a second recorder. Upon shifting to F8 I sold my original MKI to another member here. My primary reasons for moving to the F8 was the need for 8 channels with built-in preamps. Other practical features I like is the particularly compact format of the F8, and having all controls on the front of the unit. The larger brighter meters are nice, but the 680 meters were always sufficient for my use. I do prefer the F8 overall and am very satisfied having made the shift, yet there are some minor fidgety details that sort of bug me, a feeling I never had with the simpler 680s.
In these ways I actually prefer the 680MKII over the F8:
Simpler to fully understand and operate
Easier/simpler menu navigation
Better implemented easier to use channel ganging/unganging feature
digital in/out and word clock syncable
6 channel analog outputs
A more similar comparison both in channel count and cost is probably to the 6-channel F6. I've not used one.