I flew with them 5 times last year. Didn't have any issues with my backpack as long as it's not huge or overstuffed. Many people carried backpacks that are over the limit published on Flair's website but the check-in/gate agents didn't say anything.

However, be prepared to have your flight delayed/canceled last minute. I was lucky to have no problem with my 5 flights but many of my friends/co-workers spent hours at the airport waiting for their flights.