I can't answer for NCL as yet (although in three weeks I will be able to as we are leaving on the Star on 5/25). But I have looked at the reviews of cruises taken on the Paradise (CCL) and the Royal Princess (Princess) and found that often the predisposition of the reviewer was the key to the value of the review. Some of the people go on with the idea that the perfect cruise is THEIR perfect cruise. On a boat with 3000 people on it, that just isn't too realistic.
Still, I think it makes sense to pay attention to themes in the reviews and be alert to the repeating ones. For example, my wife and I know that the port times on the Hawaii cruises are short. Instead of booking our own excursions as many "experienced" cruisers do, we are opting to go through NCL this time rather than take the risk. The prior bad experiences of other cruises also made sure we didn't book rental cars through Hotwire or other entities that lock you into "no change" deals, since it is apparent that sometimes ports and times DO change.
Pasteur said that chance favors the prepared mind, and a cruiser who reads the reviews has a prepared mind and is more able to adapt to the changes and go with the flow. After all, it is not what happens on vacation that matters, it is how you react to it.