I'm definitely interested in picking one up at some point, just so I can ditch the massive old model. Keep that as a spare or something.
For those who don't know, these are the reasons to use this dongle:
1. Support for up to 8 controllers. But what PC game supports more than four players? Towerfall Ascension. There's a PC exclusive version with wider maps that support up to 8 players. Then there's the fact that you know it'll work perfectly with anything less as well.
2. It's easier to use than Bluetooth. Need to hook your friends controller up for tonights game of Gears? This dongle got you. Just hit the sync buttons and you're Goldielocks. No need to mess about in Windows menus. This is especially important for those technologically challenged.
3. It supports wireless audio. Just like on the Xbox. You can get your sound through the controller. And it'll work with headsets for chat too!
4. Rumble triggers, son. You want to get the full rumble in the jungle Forza Horizon experience? You'll need this dongle.
I feel like I should also mention that there were serious connection issues with these things almost a year ago. Mine would need to be re-plugged every time I wanted to use it. But they've been fixed, and now mine is as stable as a 3DS. (knock on wood)
No, I'm not affiliated with Microsoft.

Just letting gamers know this is the current champ of wireless controller solutions for Windows.