I have experience of a Ricoh 580 and the 680 appears to just be an updated version that appeared around 1981.
Bought it as a neglected spares/repairs watch and it had loads of movement issues... The donor I found (same movement was used by Rotary) was equally bad! Eventually managed to combine the two movements in one working watch.
Really frustrating but learned a lot in the process and the end result looks great. No complaints about the case but let down by fragile movements.
If it's cheap and working go for it. If it's got faults you might have a long wait for a donor.
A technical PDF is here:
http://www.awci.com/wp-content/uploads/watch-tech-guides/ricoh/Ricoh-680.pdf
Bought it as a neglected spares/repairs watch and it had loads of movement issues... The donor I found (same movement was used by Rotary) was equally bad! Eventually managed to combine the two movements in one working watch.
Really frustrating but learned a lot in the process and the end result looks great. No complaints about the case but let down by fragile movements.

If it's cheap and working go for it. If it's got faults you might have a long wait for a donor.
A technical PDF is here:
http://www.awci.com/wp-content/uploads/watch-tech-guides/ricoh/Ricoh-680.pdf