Hmm, I find your comments interesting since I have one of them; I bought it about 2 months ago. I'm using it with Windows 7 RC. Actually, I had not noticed the problem you mentioned because my use of the wheel has been only for vertical and horizontal scrolling and, for that, it seems to work fine. But, after reading your comments, I tried pressing/clicking the wheel to invoke the function assigned to it when used in that way and I see what you mean. It seems to not respond well when using it that way.
However, I've been experimenting some with it and I get the impression holding my finger in a certain way when pressing down on the wheel seems to affect its responsiveness. It seems that, since the wheel is designed for horizontal scrolling, pressing down to click rather than pressing sideways to scroll horizontally must be done just right because a slight sideways pressure seems to confuse the mouse.
Here's one technique that seems to work better. I hope you try it and let me know whether you notice any improvement in its responsiveness. When you want to click the wheel, arch your finger more so that it's only pressing right at the front of the wheel; your finger should be touching the mouse right up at the front end of the wheel opening, not on top of it. Then, press, or pull, somewhat backward, as opposed to a downward pressure on top of the wheel. I find that mine works better when I use that technique.