I've worked at several English schools in Taipei including Kojen. Obviously the personality of your boss is a big factor when assessing a school but with about a hundred schools they are going to have good and bad people working for them. You just have to get lucky.
I wasn't completely happy working for Kojen (for several reasons like not wanting to work Saturdays) When I talked to the people at head office about the possibility of leaving before my contract ended, they were quite fair. They were willing to transfer me to another branch if I wanted to. I didn't want to transfer and I left early but perhaps that's an option for anyone who is unhappy with their boss or workmates. Looking back I think that as far as chain schools go, Kojen is pretty good and as a newbie I benefited from my years there. You could certainly have worse introductions to the teaching scene in Taipei.

Joe