The excitement is because previously it was impossible for Android users to communicate with iPhone users over iMessage. If you're not in the US, you probably won't get this, but in the US it's very easy to be "the odd man out" as the only Android user in a group of friends. Group chats between iMessage and Android users is basically horrible right now - I seriously considered getting an iPhone solely because friends were annoyed with me for breaking their group chats.
What Beeper does (especially since it's such a huge technological achievement by reverse engineering the iMessage protocol, as opposed to just using a Mac server as a "man-in-the-middle" iMessage communicator) is unlock a part of the iOS ecosystem to non iOS users in a way that Apple has been avoiding for years.
Yes I'm not in the US. Here in France (Europe?) pretty much everyone uses Whatsapp; it's not a problem to do normal text between Android and iOS, but I'm not aware of group chats that would be exclusive to iPhone users (I have never been offered/asked to join an iMessage group).
There are tons of interoperability problems. Google put up this website a while back, before Apple announced they would support RCS interoperability with iMessage, that explains a lot of the broken experience: https://www.android.com/get-the-message/
What Beeper does (especially since it's such a huge technological achievement by reverse engineering the iMessage protocol, as opposed to just using a Mac server as a "man-in-the-middle" iMessage communicator) is unlock a part of the iOS ecosystem to non iOS users in a way that Apple has been avoiding for years.