I've been using Thor Quadrant ATV boots for since April and find them and excellent boot.
Touch wood they're waterproof, have enough flex to walk in, a decent sole so you don't slip over, and for me they'll just about fit inside a trouser leg (but then I have thin ankles). Touch wood again, I can't tell you about crash protection as I've not experienced it, but they seem sturdily padded in the right places. No doubt not quite as good as MX shelled boots, but I reckon a decent compromise.
I did find the sole would grip too much on my toothed footpegs, so I swapped the footpegs, it's one small thing to consider for any sole with a decent tread patter I guess.
They're not the only make, I guess as quadding gets ever more popular more manufacturers get on board, so it might be worth a look in a quad shop if you have any nearby.
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