Going back to the question of "what are miles worth to you" one has to consider the value of miles form one airline to the next.
For example, I am now firmly in the NW camp because I believe that their status miles, which give you (domestic) first class seating on any fare, are worth more than CO miles which do not. Also, I can upgrade to Europe for 40K roundtrip, which on CO would cost 50K, with no distinguishable difference in service (to me).
So when I started looking at miles seriously last year (yes, I'm a newbie - could you tell?) the above factors lead me to figure that I could live with as much as a nickel per status mile this year - while I build up to my first ever Platinum level. Once there, I'll figure it all differently given the 125% bonus.