I haven't really noticed it much on my 2012 Si's, but I can tell you that my 2006 was unbearably bad.
To those who take offense to those 'whining' about the rev hang, I would encourage you to drive a vehicle affected with it in the winter in snowy and icy conditions. My 2006, until it was completely warmed up, would hold onto the rev for several seconds. Or worse, sometimes it would actaully UP rev when the clutch was in. The car would buck, lurch, chug, and it was a dangerous nightmare to drive until the engine warmed up, which, when it's -32 below zero, can take some time.
I've either gotten used to it or my 2012 doesn't really do it, but I sympathize with anyone who feels it is hard to drive because of it. I went through it and yes, it was a problem for me. I can't speak to the 2012 models, though. Just wanted to share my thoughts.