We did this for the holidays in 2005/2006. While we weren't thrilled with Princess, in general... we did love the cruise itself. Seas are going to be a crapshoot, the Pacific is unpredictable as all get out.
We rode out a massive storm that slammed into California and put her in rainstorms for days. However, we did not see a drop of rain on the ship and had great weather for all 15 days - even in Hilo!! The incoming cruise before us rode in 17ft swells, we rode out 12 ft swells with crosswinds (which causes most of the seasickness). DH was in bed all of Christmas Eve which was our 2nd full sea day (just about the time we passed over the plates). The pools were closed for a couple of days heading out. You can see my photos of the pool fountains in my pics here: http://travel.webshots.com/album/550193773sVwaoh
However, on the way back - I prepared DH for the same thing coming back. We were to have a bridge tour the morning that we should have gotten the roughest seas coming back so I made him take a bonine the night before. I didn't want him missing the bridge tour. Go figure that this was the smoothest day ever. The TV screen called them "wavelets" - it was smooth as glass all the way to Ensenada!
If we were to do this again, we would do it on Royal Caribbean when they offer it a couple of times a year, or would do a 1-way. The 9+ sea days were a bit much for my husband. I loved them. If they had been broken up more, instead 4-5 day stretches, I think he would have been fine.