I use different sites for different things and also compare reviews from different sites. My most used site and favorite is TripAdvisor. I use it for attraction reviews, when deciding on locations to go to, restaurants, and hotels. I use it as a base point and compare other sites to it. The reviews on TripAdvisor are generally robust and detailed.
I use Yelp for restaurants but I always compare to TripAdvisor because Yelp reviews have a reputation of being biased towards businesses that choose to advertise with them. More users use Yelp for restaurants than other sites, so you've usually got a bigger sample size of reviews to choose from.
CruiseCritic has lots of good reviews for cruise port destinations, even if you're not visiting there from a cruise.
Travel booking sites like Expedia and Orbitz have reviews submitted by users and also photos. They're similar to TripAdvisor in format and is generally very detailed.
Travel Blogs/Social Media are great ways to look at honest experiences that are generally more personalized than some of the review sites can be. I love travel blogs for photos and videos and thorough narratives about times spent places. There are always some types of good tips. I find these generally through Google or Social Media searches.
Deciphering information & making decisions.
You'll notice on review sites that there are vastly different experiences expressed by different people. I've noticed that people will most likely leave a review if they 1) had a terrible experience or 2) had a great experience, in that order. Generally mediocre experiences won't be shared unless the person is an avid reviewer, which is helpful and sort of common. Ways to determine if a rating you're viewing is helpful or not is to determine the amount of reviews. The more reviews that are listed, the more likely that the overall rating is reliable. For example, I will chose a restaurant with a 4 star rating with 1000 reviews over a 5 star restaurant with 20 reviews. That's not guaranteeing either rating will be indicative of my actual experience there, but I'm more inclined to listen to the masses.
You'll also need to take a look at both the good and bad reviews. I usually like to get my feel of what people say was awesome, and what people say was bad. There are a lot of bad ones that I completely disregard because some people can be completely disgruntled over absolutely nothing. If someone is going on and on saying they didn't have enough ice in their drink and nothing else was bad about their meal, then I'm probably going to roll my eyes and ignore their one star rating. Just ask the server for extra ice. It must be exhausting being that angry. I've also seen a lot of reviews that are clearly bad because they had different expectations than what they should of had. If someone is reviewing a hair salon and complains about the complimentary wine but said they loved their haircut, I ignore it. I objectively look at the good and bad. There are some instances that someone has very valid complaints, but the bad might be worth overlooking. I've been to a ton of restaurants that have amazing food but the service is bad, and sometimes that's okay because I'm there for the food.
That leads me to another point: it helps in setting expectations. If I've read ahead of time that a hotel has a terrible pool, then I know not to bother going to the pool. If I've read on a review site that a restaurant is tiny with no seating, I know to plan to get that food to go. It's extremely useful and will lead to less disappointment when you know what to expect. Sometimes there are trade offs- great tacos for no AC. It happens.
Using it as a guide.
Most review sites have some type of save/bookmark option and map tools that are useful in saving your favorites. You can use the apps or websites as built in guides while you're on the road- or take that information and save to your own device however you see fit. The review sites will have addresses, phone numbers, hours, tips from users, and even photos to help you locate a place which is very useful when you're traveling in an unfamiliar environment.
Do you have any tips/experiences using review sites? Please share in the comments!