Takes about as much force as a regular button press would do so imo. Might take a bit of getting used to at first but once you've got it down, its as easy as anything. I do find myself missing my mouse when I'm at work and I have to stop myself from trying to click buttons that aren't there!
One thing that might be worth mentioning is that there are a few reports of people's middle click not functioning as expected. For example, mine will always make a click noise when pressed but will not register as an input unless I click it slightly to the right. There is a longish thread on the Logitech forums with similar reports, but I'm not sure if mine is truely faulty or if the contour of the mouse is throwing off my directional input. It was initially an issue, but I've been using it for a week now and its become second nature!