I would not be thrilled with weight gain either; I tend to binge eat when I'm bored and although I'm thin right now, it is only because I exercise a lot (and have a good metabolism I suppose); but I can pack on pounds in the winter if I'm not careful about my activity level.
The weird thing is I feel like maybe the manufacturer is lying, or misrepresenting data- are there actual blood level studies (pre & post insertion) that prove that the progestin is not absorbed systemically? My best friend has a congenital clotting disorder and for that reason has never been allowed to use birth control pills, but her OBGYN told her it would be ok to get mirena!!