Ok. I've read a LOT of upset people on hear complaining of the relevance of the FST. But in each episode, I totally see relevance.
It shows characters facing things from their past, as they did on the island, but dealing with them in different ways. It shows how the island effected each and every one of them. Ben chose Alex over power in the FST; in the OT he also declined power. Jack was finally able to confront his emotions about his father. Sayid chose a dark path in both timelines. Jin was captured in both, and he and Sun were seperated in both. Locke was left pitiful, although happy and waiting to be married.
I find the flash-sideways timeline to be, for lack of a better word, simply nice to watch.