What's the score with these custom fit shells? An extra £40 on what I was planning on spending, so not much. Is it just marketing BS or worth while?
I have a pair of Salomon custom shells. There was no extra cost* for the boot itself, but you may find you pay** to have a custom footbed fitted to yours as well.
Put simply, the footbed supports your arch, and 'fits' the way you're going to stand on the skis. Makes it super comfy, and stops any tiring in the feet. The shell of the boot is then moulded to fit your foot and calf better. If, like me, you have cycling calves, this can really help. My boots are super-comfy, often forget to undo them at the end of the day, which I think is the sign of a comfy boot. They also really help me control my skis better, as they're fitted properly to my shins, etc.
* I didn't pay for mine.
** See above.