I've never thrown up on or after a ride. The worst coaster sickness situation I ever got myself into was at Cedar Point. After an incredibly rough, headache-inducing ride on Corkscrew, I took my first (and so far, only) ride on Mantis. I was pretty cooked for about an hour after that.
The closest I've ever come to all-out sickness at a park would probably be from Kennywood's Wipeout, which I won't ride again.
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Matt Lynch
Co-Webmaster, Kennywood Boulevard
http://kennywood.coasterbuzz.com