There are two "flavors" of bivies- waterproof ones (gore-tex, eVent, others) and water resistant ones; the water resistant ones typically breathe better, but are meant to be used with a tarp (unless you are 100% sure it's not going to rain!)

Word of warning- even the water resistant ones, that breathe better, are prone to condensation in humid conditions- that was hammered into me on a backpacking trip through the desert of Joshua Tree, the last place I thought I had to worry about humidity! (effed up site selection night one, did not eff up night two and three :D)

I still use my eVent bivy, but almost exclusively during the winter and in snow shelters.

Solo shelters have gotten so light that it pretty much nixes the advantage of a tarp/bivy. This shelter, replete with a bathtub floor, bug netting and a vestibule, weighs 17 oz with stakes- tough to go back to a bivy/tarp setup.

[Linked Image from imgur.com]