If you want to log all ethminer output console on a text file, and at the same time you want to see your usual ethminer console while working, use this syntax:
on linux:
ethminer -G -F
http://yourpool 2>&1 | tee output.log
on windows:
ethminer -G -F
http://yourpool 2>&1 | wtee output.log
On windows, download wtee from
https://code.google.com/p/wintee/downloads/detail?name=wtee.exe and put it inside the ethminer directory
Logging ethminer output can be useful to check successfull submissed solutions, failed ones, gpu errors, stale, etc..