I booked our first cruise before I found Cruise Critic. I started reading the reviews, some good, some bad and a lot of things about Carnival in general. I was a little concered but it was too late to change so we went. It was a great cruise on the Carvival Conquest in 2003. We went expecting to have a good time and we did!!
The important thing to remember is that if you want to have a great time you will, if you let all the little things bother you then you won't enjoy your cruise. We have since taken 8 more cruises on Carnival and one on NCL. We didn't like NCL as much as we like Carnival but we still had a good cruise.
We have never had any major problems. We've missed a port now and then because of weather, maybe waited a bit for debarkation just little things that we really don't let bother us.
Now not all ships are the same, when I read reviews I consider what people are complaining about. Food is very subjective, we have always loved the food on Carnival, didn't care for NCL's Freestyle dining but others love that. If I see a complaint about something repeated over and over again then I think the ship has a problem. If the complaints are more like, couldn't find a deck chair, the staff wasn't friendly, wasn't enough to do then I figure it is the reviewers problem.
Hope that helps. We love cruising, hope you will too.