Well, if you wan't to be up to date, it's very important to watch every episode. Because even fillers have little things that are revelant to the plot.
Now, not saying you don't have to see S.O.S at all, but the last 15 seconds is all you need.
S.O.S is a Bernard and Rose centric episode. It's funny, because they are only recurring characters. We learn how they met, and, that, Rose has cancer, and she only has a little time left, but Bernard doesnt matter what, wnats to marry her.
This was the only remarkable twist. The on island plot is more likely a parody, it's reeally light hearted. Bernard wants to build an S.O.S sign but eventually veryone leaves him alone.
Jack and Kate decides to go out in the Jungle to force a trade(Henry for Walt).
We learn that Rose does know that Locke was in wheelchair - this one tiny bit of revelation was good too, I must say.
Then at the end, finally Michael pops up. About time.