Love your use of that song title...
I agree. Rinoa was a much more appealing character to me, after Squall began to influence her. She was, as you said, calmer and more responsible. She also seemed less argumentative ands much less selfish. I think that had as much to do with Squall as it did with the characters' situation... being forced to go forth and save the world would be pretty sobering for anyone. Also, having the added repsonsibility of bearing Edea's sorceress powers would only compound the burden she had to carry.
When you think about it... Rinoa seemed to changed more than any of the other characters...
By the way, a thread like this should probably have a spoiler warning. *Adds one*