[mlok]

Sorry but my experience is the opposite. Here in France most mobile phone contracts give you unlimited sms + mms. I never felt the need for whatsapp personally, neither do I know anyone using it. (but Viber over wifi is often used by people traveling/working abroad)

[CalRobert]

No need to be sorry, data > anecdotes. I use Whatsapp with a group of 20 friends to organize nights out and basically use it as a long-running IRC room. We also have different groups for movies, board games, etc. It's the group functionality that makes it better than SMS.