My winter hiking experience is almost all in the Adirondacks…
Fabric: nylon/gore tex
Fill: closed cell foam
Price Paid: $99
My winter hiking experience is almost all in the Adirondacks in New York and the White Mountains in New Hampshire. I have spent several weeks in the Mount Washington area in the winter with crampons and snowshoes, and I have tried a variety of ways to keep my feet warm: leather boots, plastic boots, vapor barrier socks, supergaiters, and overboots.
In my experience, overboots are the best solution for the coldest conditions, with the caveat that overboots are not suited for walking on their own, except as camp booties, because they cover the entire sole of the boot. You have to use them with crampons, snowshoes, or skis.
The Mountain Hardwear overboots are built extremely well with very tough materials and Gore-tex. They have well thought-out straps to cinch the top and over the instep to secure the boots within the overboot. There is a wide velcro opening over the area of the boot laces.
Fit can be an issue, depending on what you are looking for. These run large compared to some other brands of overboot. I prefer that because it helps adapt to a wider range of weather conditions, but it underscores the need to try them out with boots and crampons to ensure they work.
Warmth can also be an issue. If you are going to be in a deep freeze, -20 farenheit and below, there are Outdoor Research and Forty Below overboots that may work better for those conditions unless you customize. The standard insulation in the Mountain Hardwear overboots are a series of relatively thin pieces of closed cell foam, on either side of the foot and in the sole. The toes and top of the foot are not insulated, which could be problematic in very cold weather unless you customize.
My personal rule of thumb is that the overboots are good to about zero farenheit in their 'off the rack' conditions. Fortunately, these are sized large and can handle a lot of extra insulation. I have two pieces of hand-cut closed cell foam that I stuff into the overboot and that surround my feet, including the sole, sides, toes and top of the boot, an arrangement that fits well and keeps my feet warm in the worst conditions. It's the same concept as a mukluk.
One final note: keeping your feet warm involves a variety of issues, not just overboots. You need to think about the whole combination of socks, boots, and overboots you plan to use, the straps for crampons or snowshoes (rigging them very tight constricts circulation), and how much your feet sweat. Wet socks won't keep you as warm; I use vapor barrier socks over a wicking liner in sub-zero weather.
You also need to test everything in advance to make sure your feet have room in the boots; wearing extra-thick socks inside boots that don't have enough volume can cut off circulation too, and that means colder feet.