I suspect that many people who say they would never go on a cruise, either have never been on one, or the Cruise they hated was poorly selected for them. Before you look at price, think about what you want out of a cruise.
Some ships are family destinations, some are more like quiet resorts. Virgin cruise line and Viking are adults only. Do you want endless hours roasting in the sun, or would cooler weather be more your style. Think about where you might want to visit. The itinerary does not matter if you are thinking of the cruise as a slightly mobile party, it does matter is you are taking the cruise to see places you want to visit. Also look at the time in Port, this can range from just a few hours, to overnights, seldom more than a day. Our first cruise was built around riding the Yukon and White Pass Railway, if the itinerary didn't include that option, I didn't consider that cruise.
Size matters, smaller ships often visit smaller ports that large ships can't, and because there are fewer people on board you stand a better chance of "getting to know" people (perish the thought.) Large ships are generally more stable, are limited to larger ports, and can be crowded when 2,000 people all want to get off at the same time.
Think about the cabin choice. We have sailed inside - quiet, dark, great for sleeping. Ocean view, you can see but not feel the outside from the cabin (maybe a good option if you fear falling overboard.) Balcony or veranda, gives you a "private" outdoor space to watch the waves. Being stingy we have not tried a suite. Ship cabins are small, 170-200 sq. ft.. The layouts differ a little from ship to ship and line to line. The balcony was nice, but we probably spent more time on the promenade deck than we did on the balcony. Ocean view, I wouldn't do again. I have booked a balcony for next springs' cruise.
Food will vary. Some lines serve simple, less expensive, higher volume food. Some focus on higher quality. What is important to you?
Think about amenities, if you plan to ride every waterslide around, having waterslides on board is important. If not, what is important to you, and what does each ship offer.
Cruising can be a great value, when you consider the cost of accomodation, meals and transportation it can be a very economical way to travel.
If you are only looking for an escape, with meals and housekeeping, booking a cheap cruise may work for you. If you are looking for an experience beyond the most basic, buy what you want, and don't let price be the only deciding factor.