I've had an MEC overbag for about 10 years now, and used it periodically, particuarly for snowcaving, where the environment is extremely damp (proper snow caves hover around freezing temperature, they're not that cold). It has worked to keep the down in the sleeping bag dry, and my toes extra warm!
It ads about 5-1o Deg C of rating to a bag. It has a thin layer of insulation, and mine used Gore Dryloft, which may be replaced by another water resistant product now, not sure.
Here's the link to the latest version.
The overbag leaves a layer of air between the main bag and the insulation in the overbag. The insulation ads a bit of warmth, and then of course the shell makes it harder for moisture to enter, and it also makes it a bit harder to breath. That's why they used Dryloft rather than a membrane, the membrane is that much MORE harder to breath, and then you end up with the system sweating on itself.
The other good thing is that the sythetic insulation on the overbag has a small capacity to retain moisture, a bit like a wick, so the the moisture produced while sleaping, and the moisture that condenses is tended towards the outside of the bag.
MEC in Canada is like REI in the states - I don't know if there is a comparable product.
Hope this helps
