Maybe you're having trouble thinking of a way to justify spending $100 for a rodent. I was too, but I thought of a way to justify it that maybe a few of you might find helpful:
Before now, I'd never spent more than $30 on a mouse (except once, a $50 microsoft bluetooth mouse that doesn't work too well). But I was never happy with any of them. They were either uncomfortable after an hour or so of use, or stopped working after a while, or something. So, after a few months, I'd give up and buy another one. Over time, I spent probably $200 on a series of cheap mice.
When the Performance MX came out a few weeks ago, I saw all the positive reviews (from users and from professional publications and websites), and it occurred to me that maybe if I spent more for a really good mouse, I'd be happy and wouldn't have to keep buying $30 mice every few months.
As I said above, in most ways I think this is a great mouse and agree with the many other positive reviews. But I've experienced some minor issues, too.
1. When you start pushing the mouse around after it's been sitting idle - even for just a few minutes - there's an annoying one- to two-second lag before the cursor reacts. UPDATE Sept 15: Logitech tech support figured out that this was a defect. I got a replacement, and it's perfect. With Amazon, I didn't have to pay shipping for the replacement, and I didn't have to return the first mouse until after receiving the replacement. Very happy with Amazon's service.
2. At this time, the number of devices that are compatible with the unifying receiver is very small: just two keyboards and three mice. The unifying receiver would be far more valuable if there were more different types of compatible devices. For example, I would like to see a numeric keypad and also a presentation/pointer that could work with the unifying receiver - logitech makes both such devices now, but they won't work with the unifying receiver.
3. I'm probably being too picky here, but I find the "click" to be a bit tinny, and louder and less satisfying than other mice I've used. UPDATE Sept 15: the replacement mouse is worse. The left button makes the same tinny click when you press down, but when you let go it makes a louder sound, almost like a knock on a door. (The right mouse button doesn't do this. Go figure.) This gets annoying after a while. Sorry, but if I'm paying $100 for a mouse, I want it to be just about perfect.
Since almost all the other reviews of this mouse are very positive, I am sure that my experience with two of them each having a minor but annoying defect is uncommon. In absence of these apparently rare defects, this is a really great mouse.