I think whether or not you'll get sick also depends on what you're like in other situations... for instance, sometimes I get nauseous in the backseats of cars, and I always get violently nauseous in smaller boats. I can't ride backwards in a limo without getting ill. I'm just very sensitive, motion-wise.
On our last two cruises, there were nights when the boat was pitching pretty good and a lot of people seemed to be getting a little green. I ended up throwing up on the Miracle during one rocky night, despite my patch. But 99% of the time I feel fine with the patch.
If you're not the type of person to get motion sick in other situations, you'll probably be fine. Doesn't hurt to bring some backup though!
Booked on Carnival Freedom, Feb. 2012 (Cozumel, Costa Rica, Panama)
NCL Gem, Sept. 2011
NCL Jewel, Feb. 2011
NCL Dawn, Sept. 2010
NCL Spirit, Feb. 2010
NCL Gem, March 2009
RCCL Explorer of the Seas, Feb. 2008
Carnival Miracle, May 2007
Carnival Conquest, April 2006
Caribbean Princess, Feb. 2005