Most people have them, just some can't feel them at all.
Normally mine do feel like a tummy tightening, it sort of goes rock hard. Sometimes its uncomfortable, but it depends on what I'm doing at the time. For example - this morning when I pushed the trike up the road to the hairdressers, I had about 2-3, and they were uncomfy, but only cause I was trying to use my tummy muscles underneath to control Jenna on her bike.