Shoe covers are a must for keeping your toes warm in winter, but we find that Castelli's neoprene booties aren't quite up to scratch.
The problem with these booties is that they are too thin to work well at temperatures below about eight degrees celsius, but they start to feel clammy when it gets above 12. They're made from neoprene, a non-breathable rubber commonly used to insulate against the cold, but one that needs to be reasonably thick to protect your feet from the zero degree temperatures and wind chill encountered in winter. In the wet, they are better than nothing but there are better waterproofing options out there, including Castelli's Pioggia covers.
I tested the bootie against a cheaper but thicker Pro one on a freezing morning by wearing one on each foot. At the end of an hour and a half, the Castelli-clad foot was significantly colder. Now, the Castellis retail for £30, roughly double what the Pros were worth when I bought them a few years ago. And given that you can buy Endura's popular winter overshoes for £20, these Castellis seem to be expensive for what they do.
I did like the reflective strip on the back, which will help you be seen on those dark winter days and nights. You have the choice of either anthracite (dark grey) or red for colours, and five sizes are available, from S to 2XL, which should accommodate most feet. The bootie covers the ankle to provide a bit of extra warmth and the zip seems sturdy enough but just don't zip it up to the top or it will dig into your ankle.
The only advantage of a thin shoe cover is that it doesn't scuff your cranks and wear through. It also looks sleeker, if that's important to you. But overall, these are an example of form over function. What would improve them is putting more rubber around the toe and upper areas, but leaving the sides as they are.