I purchased a pair of Louis Garneau T-Covers to help keep my toes warm and dry so I can use my ventilated spring/fall mountain bike shoes through the winter.  They're perfect -- easy to slip onto the shoes, light, and -- perhaps most importantly -- they keep the wind and rain out.  I have splashed through some pretty significant puddles -- water just beads up and rolls off!

I can also get my shoes on and off without removing them.  This allows me to remove my shoes' toe spikes and install the toe covers under them for an extra-secure fit without having to do so every time I put my bike shoes on.  They can also be slipped on over the toe spikes, however, without damage, which will come in handy once the weather warms up.

Fit guidelines available on the Louis Garneau website appear to be on target.  I wear a size 41 or 42, depending on the manufacturer and how many pairs of socks I plan to cram in.  The shoes with which I use these covers are Adidas, similar to the Durango model, in a size 41.

Where warmth is concerned, the shoes in question have less breathable mesh than many cycling shoes, but would still be chilly in temperatures below 40F or so without the toe covers.  With the toe covers, I have worn them with a single thin pair of socks in temperatures as low as 19F (with a wind chill of -9) and my feet have remained quite comfortable: not hot, thank goodness, but comfortable.  There's no need to cram in multiple pairs of socks -- my toes are free to wriggle to their hearts' content!

Of note, these wouldn't be my go-to choice for the kind of ride where one is likely to find one's self ankle-deep in a stream crossing (I have both the Garneau's Neo-Protect booties and a pair of fully-waterproof Lake winter cycling boots for that, depending on what kind of riding I'm doing).  I suspect water would probably find its way in under the T-Covers.  However, for all other winter riding applications, or even rainy rides during the rest of the year, the T-Covers are the ideal answer.  They're also small and light enough to toss in a jersey pocket or a small seatpost bag against the possibility of unexpected showers.

Thus far, I am perfectly satisfied with Louis Garneau's T-Covers.  I plan to order a couple more pairs -- one for my road bike shoes and at least one "backup" pair.

Update: On a super-muddy ride, I managed to completely destroy the bottom piece that connects the sides on one of my T-Covers.  However, it's still doing the job just fine without the connector.  Awesome!