I was on the maiden voyage and don't know what ship the "nightmare" people were on.
As far as rough seas - our first full day at sea had 40 foot waves (according to the Commodore), and it LOOKED very rough, but I've felt worse seas in the Caribbean at 6 foot seas. The QM2 rides VERY smoothly, with a gentle movement during the bad seas. Several people were seasick, but for 99% of those, it was their very first time at sea, and the medication available could easily remedy problems.
It's a beautiful ship. Sure there were service problems - it was a maiden voyage. But lately, more people are saying the service problems are working themselves out... I'd sail on her again in a heartbeat.