Likable FMC Important?

There's been so much conversation about how unlikeable the FMC is from the Burn of the Everflame, that is got me to thinking. How important is it for you to like the FMC in order for you to like and recommend the book? Why is likeability important or not important? What makes you want to root for her? Is there an example of an FMC you loved, flaws and all, and wanted her to succeed with whatever the quest was?