I just picked up a DR-680 MKII to add to my existing DR-680. I'll use the MKII for most things, and use them both if I need more than 8 channels. A few notes/questions:
1) I got mine from Full Compass, and while the price on their site is $599, I asked if they could offer me a better deal, and they sold it to me for $535. I'm a semi-regular customer there, so I don't know if that comes into play at all (probably not), but definitely ask if you're thinking about getting one there (or anything else for that matter).
2) The menus seem much nicer and intuitive than the original 680. I finally upgraded to the 1.30 firmware because I knew there were some improvements, and I was a bit sad that despite a few new features, but basic layout didn't change (the original 680 still has an inferior menu layout even with the 1.30 firmware).
3) I'm planning on recording a show tomorrow with more than 8 tracks, so I wanted to see how the cascade function worked. I *thought* it has been mentioned that when using cascade, the master and slave units don't stay in perfect digital sync, but I recorded for close to 7 minutes, and there was no drift. A null test between the files recorded on the 2 recorders resulted in perfect silence. The only issue was that the slave unit (680) was 1,841 samples ahead of the master (680MKII), so I had to line them up in post. I haven't (yet) experimented to see if that offset varies, or exactly what happens when switching which unit is the master and which is the slave. I'd be interested in hearing what other peoples' experiences are, however.
4) On one hand it's nice that there's not a separate line cord that can become unplugged from the power brick, but I do wish the cord to the brick was longer.