This has been discussed extensively over the years on FT. Pick and choose any of the thread/s you want to dive into:

http://www.flyertalk.com/forum/miles...ints-then.html

http://www.flyertalk.com/forum/miles...ash-value.html

http://www.flyertalk.com/forum/miles...nus-miles.html

http://www.flyertalk.com/forum/miles...d-ft-miss.html

Welcome to FT!