Rating: 4 / 5
At one pound, this bivy sack is perfect for use with a tarp. It's made of REI's proprietary Elements material, a mostly waterproof/passably breathable material that easily handles the moisture that blows in around the edges of a tarp, and turns the dew that sometimes forms when you're sleeping under the stars. Its best feature is the opening - it opens nearly to your waist, so getting in and out is a piece of cake. Six zippers supposedly give you the option of creating arm and head holes, so you can coccoon inside it while you cook, read, or pose for the catalog pictures of you cooking or reading. I've never tried that feature; if I'm in it, I'm sleeping.
It has a mesh face opening (shoulders to top of head) that provides some ventilation; the mesh is not backed by an Elements panel, so you can't close it off. Although it could use a waist-length mesh panel, I've even used it on a muggy July night in Indiana, and haven't overheated or slept in a puddle of my own perspiration. (The extra mesh, if backed by an Elements panel, would add some weight, however.)
The weight's certainly right; combined with my Granite Gear White Lightnin' tarp, my total shelter weight is about two and a quarter pounds - and the combination gives me the option of sleeping under the stars (which I love) or being fully sheltered from any storm I'm likely to encounter in Ohio, Indiana, Kentucky, or Michigan. (Well, OK, tornadoes not so much - but certainly rain.)
For cool weather, I've found it adds about 5 degrees to the rating of my bag - saving a little more weight by letting me take a lighter bag.
7/9/07: I've downgraded this to a 4. My earlier experiences in hot, muggy weather must have been a fluke - or, more likely, this bivy paled in comparison to the Integral Designs Salathe that I now use. The thing that makes this a 4, and the Salathe a 5, is the mesh panel: the Salath'es panel goes from head to waist; the Minimalist is only a face-wide opening. The Minimalist is still a good starter bivy, and probably an excellent choice where conditions are always cool at night, but it doesn't fill my needs as well as the Salathe.