We went to Kefalonia early September 2002. It rained for practically a week but I don't think thats the norm. The rest of the time the weather was very hot! The thunderstorms are amazing and kept us entertained anyway :)
Lassi is fantastic. Its got just the right mix of things to do without being too commercial and would probably appeal to all ages. The restaurants are great and the staff really friendly. Don't go to Oscars restaurant unless you like being dragged onto the dancefloor midway through your meal! The walk along the coast to Argostoli is well worth doing. Brilliant scenery and lovely beaches. I will definately be going back.
It was certainly relaxing but when we went, the weather was wet. Its meant to be much more colourful when the sun is shining. The guide was quite funny. He kept singing and flicking water at the other boats. We didn't think to take any money down with us though and he gave us a dirty look as we walked past his upturned hand.
Don't go when its raining! Probably lovely when the sun is out but we went during a horrific thunderstorm and there weren't many places to take shelter. Some nice little shops though and well worth a look.