My old roommate and my best friend both have MS IntelliMouse Explorers (the Grey&Black ones) the buttons feel kinda mushy to me so I went Logitech, and they have a nice solid click. Using the scroll button as a click is a bit mushy, but the buttons are nice and solid. I like the feel of the scroll wheel click a bit better on the MS, but that's it. Also Logitech adds weight to the mouse so it feels as heavy as a regular mouse (those little balls are most of the weight in a regular mouse), MS doesn't so the mouse feels really light.
If you are concerned about tracking or whatever, they use the same sensor so that's a non-issue. It's just feel of the buttons, drivers and shape.