Was in Kefalonia for the first two weeks of August in 2006. The weather was great, really warm and sunny in the first week, but it cooled off towards the second week (thankfully!)! There was a nice breeze which took away the 'sting' of the sun and made it a bit more bearable. Bring lots of sun cream!!!
Beach was fantastic - a mixture of sand and shingle, but we found it to be great! Sea brilliant for swimming - you can get quite far out safely. Pleanty of space back on the sand, never felt too crowded as we have on other more popular islands. Often a nice breeze, rather than at the pool where it is normally a bit of a sun trap. Beach also not too far away from shops / cafe's, etc, for lunch and to get a drink!
We were there August 2006, and in October, we booked up again for August 2007!
Definately worth visiting / spending as much time there as possible!!