USA Today article : LOST cast members talk about Flash-Sideways

Here what fews actors and Damon have to say about the FS (Flash-Sideways)

Jorge Garcia : "I feel at some point, it's going to come to a head," "I just don't know what it's going to be."

Damon Lindelof : "There's a reason. You're supposed to be asking yourself the question of why are they showing me both shows. The fundamental mystery is what is the relationship between these two things."
"There are going to be a lot of good theories about it, we hope. That's the fun of Lost," "If you can make it another 16 episodes, your patience will be rewarded."

Evangeline Lilly : "I really love it. I love the fact that once again I get to try on a new version of Kate,"

Emilie de Ravin : "That was a big shock to me to read that side of the characters and how they're slightly different,"

Matthew Fox : "The notion that Jack both succeeded and failed at the same time was really interesting. It has great potential to be very powerful, what this other reality, 815 not crashing and landing in Los Angeles, would have meant to people we have come to know in juxtaposition to who they are on the island,"

Terry O'Quinn : the flash sideways is confusing, but he says it doesn't matter. He just wants a satisfying outcome to the series.

source : Usa today