The downward visibility depends on the model, of course. If there is a chin bubble, the visibility is excellent. Without one, it's not so good. Everything is a compromise of some sort, so the approach angle and speed has to be balanced with the ability to see where you're going. The less you can see, the slower you need to be. In the model I fly, you can see almost, but not quite, vertically, up to ~70 degrees down. I don't often get completely vertical, but sometimes it's necessary for the last 50-100 feet, depending on the obstables and LZ size.