What shoes do you have? Lake Winter Cycling Boots (MTB version)
How cold is it where you ride? I have used them when temps were in the 20s.
How much more time can you spend riding in that temp. because of your shoes? Significantly more (2+ hours)
How do they fit (relative to anything, like Sidis, like Birkenstocks, like other cycle shoes, or other street shoes)? I sized up because I wear them w/Smartwool snowboarding socks, which are thicker, and chemical toe warmers. Plus, having extra room is good for circulating the air -- keeps your feet warmer. They are clunky (Herman Munster shoes, not stylish but who cares?)
Do you also have extra insoles or booties or other tricks up your sleeve (or your sock)? The smartwool snowboarding socks are wonderful. In addition to keeping my feet warm, they keep my calves warm.
Anything else? Booties/toe covers, etc. can't compare. If you are serious about riding through the winter, good quality winter cycling boots are an excellent investment. I've had my original pair of Lakes, which I bought on sale, since 2005 or 2006, and they are still going strong, despite being worn in some cold, rainy conditions.

FYI that I also have the Sidi Diablo winter boots. Those do not come anywhere close to keeping my feet as warm as the Lakes. I had to put both toe and shoe covers over them, and they still didn't keep out the cold (remember, I'm an endurance rider, so I'm out ~2-3+ hours/ride).

I'm looking for SPD compatables. Both the Sidis and the Lakes are SPD compatible.