The latest Netcraft survey shows Apache gaining substantial share against Microsoft's web server. As you can see in the graph, iPlanet and others are sinking into oblivion. We have quite a few instances of Microsoft's web server running and it causes most of our web-based security problems. Like everyone else, apparently, we've been replacing it.
Our state de facto standard is for iPlanet, but Apache is on the emerging standards list and I've been encouraging its use in the State whenever I can. I've been an Apache user since it was first released. Like any piece of widely deployed software, it has had some security problems. The difference is that they've been geometrically smaller in number than Microsoft's and they're quickly and reliably fixed.