That's what safewords are for -- you can yell "no" all you want and still hope that it goes on. You shouldn't be tickling someone against their will.
For me, I just plain enjoy the sensations, so I don't have the urge to yell "No." I mostly yell things like "whoa" and such. I will yell "No, stop" if the tickler likes hearing that -- some do, and hey, it should be a mutual thing, right?
I would describe the feelings themselves as a mix between the feeling you get on a rollercoaster and erotic touches. But that's just me.