We went to Hawaii on Island Princess in April of 2007. The first two days were a bit rough but not enough to really bother anybody. The rest of the trip was very smooth. I monitored the Hawaii sailings for a year-and-a-half before our cruise: my conclusion was you can have smooth seas or rough seas any time of the year. But the great majority of crossings were on smooth seas.
My wife was on Golden Princess before the refurbishment and loved it. A good friend was on her right after drydock and REALLY loved it--said it was like a new ship!
We are going to Hawaii on Golden Princess in October, '10.