We are just back from a week in Agia Efimia. Wanted to share my impressions in brief. The southern part of the island isn't the best place you can go to, Everything between Scala and Argostoli, including them doesn't deserve the time spent to reach them. Scala itself is a worse place than many other greek villages.
The northern part is OK, Assos anf Fiscardo are really nice, Agia Efimia is also pretty nice place to stay at, just 10 minutes by car from Myrtos. Mellisani lake is really impressive. As a whole the island is interesting and pleasant place to visit, lack some greenery in my opinion, recommend to visit Skopelos, maybe a better place to go, with more trees and green coloron the hills, anyway, Kefalonia can offer a lot to the tourist, but not in its southern half!