This problem appears only when "desktop effects" is enabled - with out it the menu access is OK:
The problem is that for good usability you want the main menu to be very accessible. In Ubuntu's GNOME this requirement is met by positioning the menu on the top left corner of the screen making the target size to hit it infinite in size (see Fits Law). The problem is that with "desktop effects" enabled if the cursor is thrown to the top left corner then it can't activate the menu - although it looks like it should, clicking the first 2 top most pixels of the screen does not trigger the menu (or any other launcher for that matter - it might not actually hit the panel).
The result is very poor usability - a-la MS-Windows 95 through 2000 where the start button was a few pixels off from the bottom left corner - making it difficult to hit as you have to either carefully approach the menu in order to click it or throw the mouse to the top of the screen and then bring it back just a bit.
That looks like a compiz configuration issue, that's not an issue using the standard installation