I have a partner who has assgasms as you describe. When it happens, he involuntarily hyperventalates and moans, and writhes, and sometimes lets out a little yelp. The first time I topped him, I thought I killed him. I think I was his first. (He implied there were a couple of failed attempts in the past.) He said "NO! Keep going, DON'T STOP!" So I rode like the wind. So now that we've been together about a year, it's the same every time. There is a point where he just goes off the deep end, and then I pull out and lie on top of him. It's pretty amazing. I'm about 8.5" and he does love the fact that he feels I'm big. He is about size (but SO handsome and hot).
I finally convinced him to top me to see if it would happen for me. I've only been topped 2x before that. So far, I don't get the same effect as he does when I'm topped, but I've learned the complete joy of being a versatile top. Giving and receiving is amazing. I love both. I have always been scared to death of being fucked, but now I LOVE it. However, I don't know that I would allow anyone else to fuck me. I'm pretty much into it for love, not just sex.