I've only used them on one winter trip to the Adirondacks. We experienced lows of -17F, and wind speeds over 30mph on the summit of Algonquin. Being able to zip these on without taking off boots and crampons was necessary. The warmth wasn't extreme, but was enough, and the fabric cuts the wind, which was also necessary. These are probably not the best for the most extreme cold, but having full zips and synthetic insulation was a must for me.
The zippers are a bit difficult to operate at first, but after a few uses seem a bit easier. The dual fly zipper is great! Being able to zip up, rather than fish out the upper zipper from under coats and pack waist belts is wonderful. The outer material feels more durable than expected. The ankles baaaaarely fit around my double-plastic boots, so that could be an issue for those with bigger feet (I'm a size 9 US, 42 Eur).
This review was written in the old system and had content requirements that are different than reviews written today.