As I understand it the flex index number is a little like alpine resort trail ratings -- usefull for comparing runs within one resort -- but resort to resort comparisons are a bit uneven -- e.g. a number of "black" runs at the "Summit" would be "blue" runs at Crystal.
The other thing I've come to understand about the flex index is that it represents the side-to-side flex -- not the forward flex. Garmont MegaRides -- a popular AT boot has a flex index of 110 -- but has a very soft forward flex. Alpine boot flex index vary considerably -- a 130 index is typical for "expert/racer" class boots -- but there are alpine boots in the "beginner/intermediate" category with flex indexes of 80.
The new prices on AT boots is pretty intimidating, the prices on used gear is much more affordable and can be a good deal if you know what to look for, how figure whether the shell fits your foot and how to re-bake the liners.
Getting the right shell fit is fairly easy -- take the liners out of the boot and place you bare foot in the shell and slide forward until the big toe just touches the end. Then see how many fingers you can fit between your heel and the shell. No more than two and no less than 1 has word for me.
An easy way to re-bake the liners is to use a heat gun to warm up the liners in the boot and once they are soft, put your foot wearing your ski socks. This is typicially sufficient -- as long has the orginal owner and your feet aren't radically different.
A more complete way to re-bake the liners is to use the "rice-bag" method -- fill ziplock bags with rice and warm in the microwave and using oven mits place inside the boots for 10 minutes, remove the rice bags and slide you foot in and buckle lightly.
More info on boot fitting.
www.wildsnow.com/72/bootfitting-for-backcountry-skiing/
www.telemarktips.com/DrTelemark.html
www.telemarktips.com/VideoStatic/thermofit.wmv
