I use Viber since it is more common in my country. Here the SMS service is not so expensive, but the MMS is something like 0.25EUR per message. So using Viber for pictures and etc is saving money. I don`t use it for phone calls however since the voice is rubbish compared to the regular phone calls. My monthly subsription includes 600MB of traffic and reduced speed afterwards for 8 EUR + 500 free minutes.