I have been using Logitech MX Air Cordless Air Mouse for almost 2 years with my HTPC media center and have been extremely happy with it.
The problem with traditional mouse is that you need a surface like desk to operate with it. As we all know, your sofa is not really the best surface for your mouse. If you are familiar with WiiMote (remote control for the Wii console), this mouse behaves quite similarly, but you do not need any infrared receiver as you would need with Wii.
The mouse makes browsing Internet a breeze as you can e.g. zoom in and out text size by simply waving your hand to left or right. Similarly volume control works with gestures. Air Mouse works well in both PC and Mac environments.
The Logitech Control Center allows you to customize the button behavior and other settings. A useful approach is to customize program specific buttons, e.g. if you want to optimize buttons for specific media center software. In Plex Media Center, I programmed mouse’s back button as ESC key and left click as ENTER key. Similarly, I changed cruise buttons as up and down keys so that navigation worked better and got more remote control type of feel. With this approach you can make your mouse available even if the media center software does not support the mouse. E.g. FrontRow does not support mouse by default, but by modifying keystrokes as buttons, you can use your Air Mouse.
Another benefit over infrared remote is that you do not need line-of-sight to operate. This makes controlling your computer from a distance easy. However, the range is not very long if you have obstructions on way. To overcome this, I attached a small USB-hub under my TV, where I inserted the Logitech USB-dongle. This way the USB-dongle is nicely hidden, but you still get a good range.
You can also place the stylish charger docking station next to the television so it is easy to store the mouse when you are not using it. This way your mouse is always charged when you need one. Generally, I am able to use the mouse couple of days without charging, but I still would recommend charging it every night in the docking station.
All in all, the Logitech MX Air Mouse is the best HTPC mouse I have ever own. My wife even wanted to buy another MX Air mouse as a back up as our more than 2 year old mouse is giving signs of aging.