We hired last year for a few days at each end of the holiday instead of paying for transfers and this year we've got a jeep for the fortnight.
It's not really a problem. We drove into Rethymnon and along the coast to Frangokastello as well as several shorter trips and didn't have any problems really. The locals are a bit relaxed about double white lines and seem to ignore them and overtake if there's room.
The road to Frangokastello was interesting last year. It's a beautiful drive but Rodakino was being dug up from one end to the other to put in a new bridge over the gorge so that was a bit hairy and the Greeks aren't big on barriers on the side of the road, so in quite a few places the side of the road stops at quite a large drop. The wife wasn't too chuffed at this, but I've promised to let her drive this time, so that should solve that problem.
We were there in June last year and also will be there in June this year and the roads (except Rethymnon and around the airport) were really quiet. Much nicer than driving in the UK!