My early contractions with #1 were barely felt at all. It was just a mild tightening that I could barely feel. I was heading to the hospital for PPROM at 29 weeks at the time. They put me on a monitor and then the nurse came in and said "Are you feeling that?" and I told her I was maybe feeling something. She said they were contractions. The monitor was picking them up pretty well, but I could barely feel them at all. Now after they stopped labor and then took me off drugs a few days later, my contractions started up feeling more like gas cramps... a bit of a sharp pain. But that labor only lasted 2.5 hours, so I think I just jumped into painful contractions without a warning!
My early contractions with #2 felt like AF cramps. They were pretty mild - didn't really hurt. Finally, I timed them, and hey, they were coming every 5 minutes!
The intensity increased over the next few hours, but they never got that sharp pain that I'd had with #1. It was a very comfortable labor (5 hours total).