I tried them for a bit- The pointy soles of sidis or similar shoes will chew up the underside of the booties in no time if you walk in them and you're still loosing a bunch of heat through the bottom of the sole, so you will still need thick socks.

That said, they're much, much warmer than mesh cycling shoes without them, at least in dry weather.
In wet weather, the water seemed to work it's way in regardless, though the mec ones look like they might be better in that respect.