I also signed up. Still using my BlackBerry Q10 as a secondary device and learned to like BBM back on the Bold 9900.
Looking forward to get it working on my Lumia, I am glad BBRY is trying to implement a Modern UI like interface instead of just porting over their BB10 interface.
People, please keep in mind this is the first public beta version, so it is quite understandable it won't be as feature rich as the almost one year old Android or iOS versions, let alone the native BB10 version.
As stated in the BB Beta blog, it will cover only basic chat features, more advanced stuff like BBM Channels, sticker packs, video calling and so on will be added later. Something similar again happened a year ago when they released BBM on the two major platforms.
As to the guys wondering what BBM is about or what makes it special:
BBM is virtually just another IM app, however unlike Whatsapp, which uploads and uses your contact list and the saved phone numbers, BBM uses a PIN based system. So you get your PIN and you can share it with,others, you'll also need your friends PINs in order to add them to your contact list. While this is much less convenient than Whatsapp, it is much more private, since you can fully control yourself which people you add to your BBM list and more important who adds you to their list.
Last but not least BBM features true sent/recieved/read indicators. Whatsapp for example only indicates,transferred to server, transferred to,phone. You can't check whether your buddy read your message or not.