There's group conversation and location sharing support. You can see who's read your messages, who's using Messenger, who's active on Facebook, and who's who in general. You can have notifications on and off at your will, and keep your conversation history and contacts backed on your phone.
The app looks exactly how the screenshots depict it, and runs very smoothly. It's separated in two screens - "recent" and "messenger". There's a minimum amount of buttons to navigate and text your way through the app. There's also a very handy feature that lets you browse your friend list by the first letter of your contacts' names.
However, if you don't have friends that start with "A", for example, the app won't waste your time with an empty screen, but it will visually indicate there are no A-names to be found in your address book.
Overall, Facebook Messenger was worth the wait. It's polished, lightweight, and shows Facebook gave it the necessary attention. You can download it from the link below.
Download FB Messenger: Windows Phone 8