Yeah, perhaps I should have been more clear about what kind of bivy, with the whole bivy shelter vs. bivy bag.
Never used either of them. But if I do get one, I will go for the bag. Keeping bugs out is a plus, as is the added protection from wind, rain, etc. The bivy shelter seems like it's just like a really, really small tent. One of the reasons why I opt for a large tarp is for the space inside, especially to keep gear out of the rain.
Another issue that I have noticed is that you guys seem to all be talking about tarps differently than I am. I checked out some pics and other threads and stuff, and what I gather is that by "tarp" most of you mean your rain poncho or a very, very small tarp. Most of you guys use stakes and guy lines and such. I only use rope or build an improvised shelter to put my tarp on top of after.
Most of the time just tying up a rope between two trees works great for me. If it is really windy or rainy I will get lots/rocks to weigh down the tarp.
Maybe I ought to check out this whole poncho tarp thing too... but then again, there is the old saying of "if it ain't broke, don't try and fix it."