I've been using a WMO for years now and have become quite fond of its shape and low heft, but the time has come that I need a side-button. The IM1.1 seems like the most logical option, but the trouble is that no place around here sells them and as a result I cannot test one. Therefore, I enlist the help of those who have used both mouses and ask the following questions before ordering one from the internet:

1) How similar are their shapes, aside from the side button?
2) Is the IM1.1 noticeably heavier, if at all?
3) To the button presses feel the same?

Thanks for your help.