In places with no biting insects and it doesn't rain too often I like a bivy best. I sleep on top of it unless it is cold or rain starts. I do carry a tiny tarp (5'x5') for rainy nights. It is easy to climb in and zip up if conditions change. This setup is no fun if it will be hot and buggy though.
I like my bivy too, but only use it for short tours and for areas that I feel like there will be no/little bugs. Or if the forecast is dry. I wanted to use my bivy for my recent tour last week through Washington. But the forecast called for wet weather, and it pretty much rained every night that I camped. So I brought my small one-person tent. And while the NW isn't as buggy as say the Midwest or South, there were mosquitoes at a couple campgrounds. With the tent I had somewhere to hide from them.
The one plus about bivys that I really like is the ability to look up at the stars while in bed.