1) If this is your first cruise, I suggest booking shore excursions through the cruise line: The difference in price isn't necessarily that great, and worth less stress. I will say this much -- don't get too carried away with excursions -- there's plenty to do and see on the ship, and just getting off and wandering around the town is great (Skagway is perfect for this).
2) Don't pay too much attention to "reader reviews." There tends to be a tendency for people to review cruises only if they are chronic complainers. A lot tends to be related to expectations. The "Motel 6" statement is laughable. My husband and I have taken 2 cruises on NCL and had a fabulous time on both (our third will be to Alaska on the Wind this September!). We've also taken 2 cruises on Holland American, one on Celebrity, one on Princess, and 1 on Crystal. I'll tell you up front that I haven't had a bad cruise. Crystal is much more expensive (I didn't pay for it, thank goodness), and it was great, but I wouldn't rank it as the best value for the dollar -- that honor belongs to NCL.
Have a great time, don't worry, and enjoy the scenery!