Some of you people's have crazy math. I've done my own math based on my usage of soley Photoshop CS.
Initial Photoshop CS4 purchase: $699, released October 2008.
Upgrade pricing for Photoshop CS5: $199, April 2010.
Upgrade pricing for Photoshop CS6: $199, released May 2012.
Expected usage, in months, for CS6 for release date: 15 months.
From October 2008, to May 2012, is 44 months. Add in my expected usage of 15 months for CS6, brings me 59 months.
So what I've spent, actually comes to $18.59 (($699+$199+199)/59) per month. Adobe wins by charging me $19.99, but not by much.
So this subscription might be good. Their challenge is to get people over the psychological hurdle of feeling like they have another bill to pay every month.