We are going on the Aug 26 Dawn Princess but are spending an additional 2 weeks exploring Alaska on our own. We have not been but have been planning this trip for over 2 years so have done extensive research including buying just about every travel book around, talking to everyone we know who's been and following this web sight all last season. Our conclusion is that September is a great time, if not the best time, to go! The weather can get just as cruddy in June as September.
Apparently the only down side is that it's a little cooler and the days are shorter, although still plenty long. Oh yes, you don't get quite as close to the glaciers because they are calving heavily from the summer thaw (but then that could be good too!).
The up side, on the other hand, is that:
1. There are less cruises so the crowds are smaller.
2. School is back in so there are almost no kids.
3. It's getting late in the season so the bargains can be significant (up to 75%!).
4. The mosquitos are gone.
5. Fall color is out.
6. The aurora borealis is back.
It was good enough to sell us. You're going to have a great trip, I can feel it!