We went RT San Diego to Hawaii on the HAL Statendam in March of 2003, and are going on the Island Princess RT to Hawaii out of San Pedro this November on the Island Princess. We love Hawaii, and also like days as sea. Since we live in So. Cal., these trips also mean no flying, so we arrive home completely relaxed. Since my mother uses a power wheelchair, it also means she can take this easily without having to worry about the airlines damaging her chair (common). Previously we crossed from San Francisco to Sydney via Hawaii on the Crystal Harmony (one way) in 2000.
The Pacific is the Pacific. You never know when it will be calm or rough on this route. Summer can be calm or rough, as can winter. Expect cooler weather the closer you are to the mainland, with it warming up as you get closer to Hawaii. It can be rough around Hawaii, esp. in the Molokai channel (ask a sailor about this!). Be prepared for some seas, and if you are lucky, it will be pretty calm.