Just like Yelp reviews give you a glimpse at what kind of experience you'll have at a restaurant, TripAdvisor now collects airline reviews that you can use when booking with a new airline. You'll get an idea of what you're in for so you can be prepared to deal with it, or decide not to book with that airline. You might always fly with the same airlines because you're part of a loyalty program, but many airlines (especially new ones) offer fare deals that seem too good to pass up, and TripAdvisor's new airlines reviews let you see what you're in for if you decide to take the bait and go for the bargain.
Beyond written reviews, you can see ratings about cleanliness, seat comfort, value, customer service and so on. Of course, you should take the reviews with a grain of salt since those who have bad experiences are more motivated to write up a review, but even those can help you be more prepared for what you'll experience. You can check out the new airline reviews here.