I began studying and designing computer mice at Digital Equipment Corporation about 18 years ago. While I no longer design mice for a living, I've tried a lot of mice over the years with an eye toward what offers the best user experience.
In order to track an object smoothly on a computer screen, you need to update the mouse position at least once per frame time. On an 80Hz screen, that means at least 80 updates per second for both the cursor position, and scrolling position. Apple's Mighty Mouse got this right. For the first time, many could scroll smoothly and effortlessly in any direction. But the Mighty Mouse has its limits.
As one of my own mice failed recently, I picked up a Logitech VX Nano. What a surprise, this mouse is amazing. It's quick, responsive, offers high precision tracking, and is smooth as silk. The nano USB receiver is so small you can plug it in once and forget it. There are no wires or dongles to get in your way, and battery life is a respectable 6 months.
The scroll wheel tilts for simple horizontal scrolling, which is not quite as elegant as Apple's omni-directional scroll ball, but let's face it, most of us don't use horizontal scrolling all that often, and a smooth responsive vertical scroll wheel is worth a lot. The tilt control is just enough to slide the part of a window that falls off the edge into view when you need it.
If this is all you want, the Logitech v450 Nano does this beautifully and provides a slightly nicer grip as well as the ability to run for over a year on a single set of batteries. The difference in shape is subtle, but the v450 Nano has a slighter wider rubberized indentation for your thumb and fingers to hold the mouse which I found pleasing. The scroll wheel itself acts as the center button. The v450 Nano also uses a pair of AA vs AAA batteries making the mouse feel slightly back heavy when you go to lift it.
The VX Nano on the other hand has a nicer scroll wheel and two extra buttons along the left top edge. Both mice are superb, but having found the VX Nano on sale for close to the same price, I prefer it slightly.
The scroll wheel on the VX Nano toggles between two modes when you press it down: click detent, and friction free. Both are smooth and comfortable, but when you want to scroll through a long document, friction free lets you flick the scroll wheel at whatever speed you want, and it just keeps going. Since pushing the scroll wheel switches modes, there's a separate center button just behind the scroll wheel.
The VX Nano also features two small buttons along the left top edge that are ideal for "forward" and "back" . If you are right handed, just slide your first finger over slightly to feel them. Otherwise the mouse is ambidextrous.
There are lots of little details that make this mouse a pleasure to use including the comfortable shape, rubberized texture, and balanced weight. The wireless is plug-and-play. I even plugged two mice into the same computer. They worked seamlessly except for getting the machine to sleep. Logitech's LCC software allows you customize the buttons and scrolling behavior while keeping things simple.
There you have it, the best mouse I ever used. It combines wireless portability with the responsiveness and comfort of a desktop mouse. There's no Bluetooth pairing needed, and no significant delays or drop outs. If you're in the market for a better mouse, check it out.
Update: What about Apple's Magic Mouse?
I tried one of these at an Apple Store recently.
I really like that there are no moving parts to wear or collect dirt, and the touch surface recognizes some useful gestures. Certainly Apple's best mouse so far. The overall responsiveness did not feel quite as crisp to me as the Logitech VX Nano, but it wasn't bad (the Magic Mouse uses Bluetooth versus dedicated 2.1 GHz RF). For anything but gaming, this could be a decent mouse with some cool features.
I didn't find the shape quite as comfortable in hand. The edges feel low and hard when you go to pick it up.
There's no way to press two buttons simultaneously (a software issue that should be correctable).
The front-to-back orientation is not obvious, although this shouldn't be a problem in most cases.