Waterproof Sleeping Bags

A lot of the bags out there have a just a water resistant shell but I was wondering what the advantages or disadvantages to having a bag like the shield series from Big Agnes or DriZone from Sierra. I would like to have a do it all bag, light, warm and if possible waterproof. Any recommendations?

The bag has to breathe to get rid of the moisture you sweat off (excuusse me, lose through normal perspiration) during the night, somewhere between 1/2 liter and a liter. This will condense on the inside of the waterproof layer, even a somewhat breathable layer like Goretex or the Goretex clones. I've personally had that happen with Driloft, a fairly breathable waterproof version of Goretex. You will get lots of condensation on a humid night, whether warm or cold.

Your better alternative is to get a waterproof/breathable bivy sack, if you are going to go without a tent. Clearly, you don't need waterproof if you are in a tent or under a full-coverage tarp. If you insist on a waterproof shell, at least use a vapor-barrier liner to keep the condensation out of the insulation of your bag. Even with a fairly breathable shell (Pertex or plain nylon, for example) you will get some condensation on cool nights.