It does make you clammy...which is why they're not popular despite their benefits.

The benefits far outweigh any discomfort. They keep vapor from condensing and freezing in the bag, they help prevent moisture loss at night as when skin is moist (or at least in a super high humidity environment), the body won't sweat as much, retaining it where it's needed, it also keeps the bag 'fresh' by keeping smelly sweat out of it.

I sweat...a lot. My feet sweat so bad that if I don't use Drysol (prescription strength anti-perspirant 20-30 times stronger than over the counter stuff) to prevent them from sweating the sweat glands rupture due to over activity (Then I have to the other prescription for Cortizone cream to keep my feet from cracking and bleeding). All my life I struggled with my feet freezing in even mildly cold temperatures...it's one of th reasons I gave up skiing when I was younger. Managing the sweat helped an amazing amount. Medication, wearing proper socks in proper layers, etc. When I started wearing the vapor barrier booties, all of a sudden my feet were NEVER cold (I first tested with bread bags based on internet research). When I realized how well it worked, I ponied up for the silnylon version from Integral Designs ( http://www.integraldesigns.com/product_d...producttypeid=1 ).

In the past I've had similar issues with down bags. With a modern synthetic bag the loft itself is waterproof and can still insulate when wet...but in the winter this moisture freezes and becomes dead weight. I've had this happen before too though it's far less common in only moderately cold temperatures. The Integral Designs vapor barrier for long bags is upwards of 60 bucks so I'm DIYing it by slightly modifying an AMK 2.0 Bivy to do the job. It won't be perfectly sealed at the edges but it should be sufficient...and has the added benefit of being a very efficient heat reflector as well.

To combat the clammyness I've dedicated some MEC silkweight micro wool long johns, long sleeve shirt and some fox sock liners to sleep wear. Very lightweight and compressable, they add very little weight to the system (handy to have around as backup clothing too). They add insulation and wick moisture away from my body so cut down on the clamminess. The wool doesn't hold water very well and even if it does, it's dedicated to sleep so it's a minor inconveniance...I'd rather have a few ounces of wool frozen than my entire sleeping bag. I keep them in a ziplock to prevent the moisture (and potentially stink) from getting into other stuff in the system.

Below are a couple of sites with information about vapor barriers and their history. Some of it might sound like voodoo science but regardless, it works.

http://www.warmlite.com/vb.htm
http://www.netbackpacking.com/vapor-barrier-and-radiant-barrier-liners.html