I had a S&S when I was 4 days overdue with DD. FOr me, it was very painful. It was 3pm when I had it done, and I don't know what I was expecting, but I was a little let down come 9:30pm and nothing was happening. My sister had called me earlier in the day and told me that every time she had a S&S she had gone into labour a few hours earlier.. so I guess I just assumed I would be the same.
I woke up 3am the next morning in pretty well established labour- contractions coming at me 4 mins apart and lasting 1-2 minutes. I don't know if it was coincidental or the S&S caused labour to begin.. (I *still* maintain it was the trauma of watching Patrick die on Offspring! )