I've used the Minimalist,and it's my second-favorite bivy, after the Integral Designs Salathe. I like the Salathe because it has a waist-length mesh panel, which adds a lot of ventilation; the panel is covered by a full weatherproof flap, which completely closes off the mesh (no open face like the Minimalist.) However, this comes at a cost: the Salathe is a pound heavier and a couple of hundred dollars more expensive than the Minimalist. I used the Salathe once without a tarp (surprise rain storm) and stayed completely dry.

I never used the Minimalist without a tarp, because of the open face, so I can't absolutely say it's waterproof. Besides, in a prolonged, 8-hour heavy rain, I wouldn't trust any bivy not to let water sneak in somewhere; it's just the nature of the beast that, unprotected, you'll let water in anytime you get in or out. I always carried a 6x8 or 8x10 silnylon tarp - half a pound or so, and pitched it A-frames style; the bivy was a defense against bugs and against the light mist or rain that blew or spattered in under the tarp. I also used a tarp for reasons unconnected to the weatherproofness of the bivy: it gave me a protected place to cook. It gave me a dry place to get in and out of the bivy, without letting rain in while I did so. It also gave me a dry place to change clothes, sit around, pack and unpack (The tarp got taken down last, stored in an outside pocket of the pack, and the hiking poles I used to pitch it went in my hands - the inside of the pack stayed dry. It went exactly opposite when pitching the tarp.)

Back when I used a tarp and bivy, I saved 2 - 3 pounds over the tents that were then available. Nowadays, with the 2-pound solo tents like the Fly Creek, Carbon Reflex, and any model Tarptent, I no longer use a tarp; the tents are just too much more convenient.