I have a pair of MUCKS for probably 5 years now. I tend to only use them down to about 40 degrees, and with a pair of heavyweight Alpaca socks. I like them.
They make a more heavily insulated version than mine, which should be quite a bit warmer.
For temperatures below 20 degrees, I use Rocky 1000 gram thinsulate boots.
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Basically same as the $150 muckers with neoprne uppers and shoe is flexible.Upper calf height,can use as snake boots.
muck boots company has been bought and sold a few times in the past few years so you might also look at other brands as they have declined a bit in quality