Agreed, we pay for each SMS, and BBM got us hooked on near-limitless chatting for cents, but was platform exclusive. With whatsapp no such problem.
Sending media/voice-notes is much slicker than MMS.
WhatsApp is a central place I can contact 99% of my contacts, they're not spread accross bbm/facebook/msn/hangouts/jabber/skype/blah blah blah. Around here, everyone has whatsapp, including my mom, dad, and grandfather... they have none of the others above.
With this kind of penetration and ease-of-use, group chats are a doddle. I'm on a friend-group that has been going for years.
Most importantly though:
I don't have to "add" people via some other means, invite them, know their username/bbm code/etc. This imho is what makes WhatsApp so pervasive (at least around here). If you add a number to your phone, you get the whatsapp user for free. No muss, no fuss. Yes I could use some other IM thing, persuade a lot of friends to use it too, but my contact list would be a fraction of what it is right now in WA, because I'd have to take the effort to "re-add" everyone that I already have saved in my phone.