Rating: 4 / 5
I've been using the Crescent Moon Neoprene Overbooties throughout the winter of 2006/2007. They are made of 6mm thick stretch neoprene that you put over the toe of your shoe and then pull back, placing the strap under your instep and using the velcro to attach around the back of the shoe. They cost around $38 (usd).
Sizing: They are sold as "one size fits all" and fit around my size 10.5 Montrail trail runners (both Hardrocks and Susitnas) very well. I'm afraid they would be too large for people with smaller feet (about size 8 mens). One issue is that when they get cold, they do not stretch as well, so if you take them off in the backcountry, they will be more difficult to put back on. I found this out during a solo bivy in December that was minus twenty when I awoke. The booties had been left out of my sleeping bag and were impossible to stretch all the way around my shoes until I rewarmed them in my coat. I'll sleep with them next time.
Warmth/protection: For the weight the warmth is excellent and they also protect the feet from wind and wetness. My shoes have stayed much drier due to this combination. I can actually wear uninsulated trail runners for multiple days in cold, snowy conditions without losing my toes!
Other: These were designed to wear while snowshoing, and for that purpose they are excellent. I've also tried to run and hike without snowshoes while wearing these booties, but the motion of the foot along the packed snow pulls the toe of the booties up over the toe of the shoe and then proceeds to fill with snow. I've attempted to make mods but have so far been unsuccessful. I will next try to glue them onto the toe of my shoe...
Pros: Light, compact, warm and protective.
Cons: Hard to put on when cold, rides up when not wearing snowshoes, sizing may not work for all.
Followup: ShooGlue on the toe area kept them connected to my running shoes for 40 miles and counting. Feet toasty warm at -30 (f).