I'm a geezer. I'll be strictly objective, but my perspective is perhaps unlike many others. I also tend to seek more active pursuits, so probably not the best candidate for cruising. With those caveats, here are my impressions of the good and bad, with particular focus on Princess:
Our cruise was the inside passage departing Vancouver June 25 2012, ports being Ketchikan, Juneau, Skagway and finishing in Anchorage.
The sights are every bit as impressive as they say, and more. The experience is not to be missed.
The cruise lines (and others) offer every imaginable excursion activity. We took the zip line tour to Grizzly Falls in Skagway (Probably the best & the longest zip lines for the money). This was the absolute highlight of the adventure for me. You'll have to book Grizzly directly. The cruise lines don't list it. (We also happened to get very lucky indeed with weather: blue sky, sun and not a drop of drizzle.) The landscape here is simply stunning.
In Juneau we took the float plane ride over the glaciers. (Booked through Princess.) Wonderful, wonderful experience. Much better than the helicopter that just dumps you on the glacier to walk around on the ice.
Onboard, the cabin stewards generally live up to their reputation for being very friendly, attentive and helpful. This adds greatly to the level of comfort. The staterooms are lovely and due to the reduced level of bookings we got easily upgraded to a balcony a week or so ahead of sailing.
If you're looking to get away from the maddening crowds, cruising is the last choice for you. Modern cruise ships are enormous & the carriers work diligently to put a paying passenger in every available square foot. If you're looking to find a comfy seat in a quiet spot to read or relax, you'll almost certainly find a body already in it. If you head to the gym at typical times, you'll find it fairly well equipped, but a body occupying every station much of the time. (A real zoo.) If you want to pay even more money than you forked over in advance, Princess has a few premium relaxation areas you must pay to enter ($20 a day) where (hopefully) you'll find a spot. Like timeshare locusts, they seem to hawk overpriced premium items (photos, bottled water, wine, soft drinks, etc) more or less constantly, which is truly annoying. The spa services are priced at roughly 4-5 times the going rates elsewhere.
Our cruise happened to have a very talented comedian-musician ("Sarge") who was clearly well above cruise ship caliber. Very funny and the crowd loved him. Much of the other talent also seemed generally well above what one might expect.
The serving & other staff is spread thin & it shows. The buffet food is, well, buffet food. If your thing is force-feeding yourself an endless variety of mediocre-at-best, nutritionally questionable calories, you'll love it. The emphasis on economy is everywhere: in the powder mix iced tea, the heavily watered-down sauces, soups, etc, as well as the dreary attitude of some staff. At times, you'll wait far longer here for a coffee or water refill than you would in your local Denny's. In fairness, service in some areas of the buffet is much better than others.
The dining rooms offer better fare, but nothing you'll rave about. Cheap cuts of meat, very small portions. Same watery sauces, soups (ugh!), etc. I got a lobster tail that looked more like a large shrimp, and it was just too scary-looking to eat.
One of our party complained bitterly about having notes to the cruise director simply ignored when she sought help with onboard activities. No response. Zippo, nada.
There is fastidious attention to cleanliness and disease prevention. I saw nothing anywhere that raised any concerns.
Bottom line, it was about equal to my expectations. The total cost was about as budgeted. For extras, expect to pay at least a thousand or two over & above your cruise & air fare. (Excursions, tourist crap, drinks, ship gratuity & other charges, gambling, etc.) Would I do it again? No, but then it was always something I thought should be done at least once, and I still do.