Pain. IntelliSense is handy in a number of ways. One way it is not so handy is that when it pops up, it covers up whatever is down and to the right of where you are typing. Murphy's Law dictates that whatever is covered up is exactly the context that you need to see in order to write your code. Sigh.
For example consider the situation shown here. I was counting on the adjacent code to jog my memory if the class member I am about to create should public, protected, internal, or private.
Band-aid. I've always worked around this annoying 'IntelliSense-makes-a-better-door-than-window' problem by dismissing the IntelliSense window by pressing the Esc key, looking at that code I need to see, and then bringing the IntelliSense window back by pressing Ctrl+J. It's not pretty, but it works. I've done this so much that I find it is hardwired into my fingers. But it is a band-aid solution at best.
Relief. When IntelliSense is displayed, you simply hold down the Ctrl key and the IntelliSense window becomes semi-transparent (is that translucent?) so you can see right through it. It's not gone: if you look closely at this screen shot, you'll see the ghost of IntelliSense lingering just inside the range of barely perceptible. Release the Ctrl key and the IntelliSense window is opaque once again.
Nice. Nice. Very nice.
Updated: as Rob comments, this is a Visual Studio 2008 feature. Sorry for not making that clear. Another good reason to upgrade.