We just returned from an Alaska cruise (review at http://cruiseline.com/ship/celebrity-solstice/review/427637 ). Had an absolutely wonderful time.
The best advice I can give is to pack warmer clothes than you think you will need and make sure everything is waterproof. The temps in late August were 30 - 50 degrees F, with rain most of the time. The good part of this was that the infamous CHOG was nowhere to be found!
Although it was an outstanding experience, In my own unsolicited opinion, I believe Princess offers a slightly better product than Celebrity simply due to ship design.
Although I missed reading Cruiseline, I can't honestly say I'm glad to be back. I'd much rather be back on the ship