I have used a few. My thoughts on the ones I have used follow:
REI Minimalist Bivy Sack - Kind of heavy at about 1 pound, but still lighter than the ones that are really small tents. It was kind of uncomfortable when it was hot, but in warm weather I slept on top of it unless the bugs were biting or it rained. I do remember one miserable night in Louisiana when it was hot and the skeeters were really bad, but overall it was fine on the trips where I used it.
Borah side zipper ultralight bivy - Much lighter at 7 ounces, packs small, and I found it to be more comfortable when the weather was sticky.
Ti Goat Ptramigan Bug Bivy - Lighter yet, packs smaller, and nice and comfy in hot sticky weather. It is my first choice if I expect it to be hot and buggy, but if a cold wind is likely the Borah offers more protection from the wind chill.
If it isn't wet, cold, or buggy I usually sleep on top of the bivy until it either gets chilly, the bugs start biting, or rain falls.
In all cases unless I expect to be able to sleep under some kind of shelter (bridge, picnic pavilion, etc.) if it rains, I take some kind of light tarp. My 7 ounce Integral Designs Siltarp 1 has served me well, but is kind of skimpy in coverage. If I need to pitch it in wet weather I have had to pitch it really low to get decent coverage. I recently got a bigger tarp (Sea2Summit Escapist 12.3 ounces). It is heavier, but if I will actually be pitching it much for wet weather it is really nice.
If wet weather is unlikely any given night I just keep my gear close and the tarp handy. Then if it rains I pull it over myself and my gear.
