Some FF is really well done. Some is really crappy. You just have to read each story and decide for yourself.
Some of the more popular 24 FF stories are:
Tony & Michelle
Jack & Audrey
Jack & Chloe
Another 24 hours of terror
A common thing for FF writers to do is intentionally break from the timeline. In the story I'm writing, I will break with the official 24 timeline at the end of episode 4.24.