By few do you mean everyone that owns an iPhone?
iMessage isn´t that popular outside the united states. I think it looks and works great, but I don´t have a single friend/family member/acquaintance that uses imessages instead of whatsapp here in Argentina. Not being able to message with android users kinda defeats the point of a message platform. Bear in mind that standart text messages are still very expensive in a lot of places...
I´m from Argentina, by the way