We took a September cruise on the Celebrity Millennium from Seward (Anchorage) to Vancouver, and it was outstanding.

Advantages:

On one-way tours, you see more of Alaska than you would on a round-trip tour from Vancouver or Seattle. Flying to/from Anchorage, though, is longer and more expensive than to/from Vancouver or Seattle.

Starting (or ending) in Alaska allows a visit to Denali (Mt. McKinley) National Park. We signed up for a two-night stay at Princess' Denali Lodge, which included travel to and from Anchorage on their private rail car attached to the Alaska Railroad train. Spectacular scenery!

You don't have to cruise on Princess to stay in their lodges, although we like Princess cruises, and booking a Princess cruise tour means that they'll handle all of the transfers and luggage. You also don't have to sign up for their bus tour inside the park itself — it costs about double what the National Park Service's own tour costs, and the NPS drivers can give you lots of information about what you're seeing.

We stayed in Anchorage for a couple of days before going off to Denali — there's plenty to see and do there.

The Celebrity ship had no children's program, so there were no children on the cruise — much quieter that way. That ship had a naturalist on board who gave interesting lectures and narrated our amazing visit to Hubbard Glacier — we were there instead of Glacier Bay, and while I've not been to the bay, I don't feel cheated, as the Hubbard was spectacular.

Disadvantages:

The ship stopped at Icy Straits Point, an old cannery rebuilt solely as a shopping destination for cruise passengers. It wasn't worth leaving the ship.

September cruising means potential weather issues. Our day sailing through the Inside Passage was gray and rainy -- we couldn't see much of anything. OTOH it was a lot cheaper than high season would have been.

Have a great trip!