Not my cup of tea but I have the Titanium Goat Ptarmigan. Got that for some experimenting...
First one should decide what type to get according to the intended use.
For example the TiGoat is an ultralite (about 180g) breathable type (20 denier "Intrepid" at the top, silverised silnylon at the bottom) designed to work under a tarp beign only splash proof.
At the other end you have the waterproof Goretex 700g plus ( Oudoor Research/Black Diamond) or the more breathable (according to most reports) eVent types ( Integral Design)
These ones are often used alone or sometime inside mountaineering tents for extra warmth.
Black Diamond have Epic bivvies , they breathe well but are only water resistant. So not good under prolonged heavy rains.
The hooped/tripod types are a bit less claustrophobic and allow some degree of maneuverability for reading or changing.
Something to keep in mind is the mat that you intend to use and if you prefer it inside or outside the bivvy. Setting up your system (mat , sleeping bag and you inside) and running a tape measure around it will provably reveal that you need a larger bivvy than you thought...
Hint : some dead air space between your sleeping bag and the bivy is better than compressing your bag.
Franco
some info here
http://www.prolitegear.com/site/xdpy/kb ... index.htmland here
http://www.backpackgeartest.org/reviews ... d%20Bivys/