Whatsapp API experiences?

I am investigating how to add Whatsapp messaging in a web application. I have seen that a few companies, such as twilio, voximplant or Clickatell (their prerequisites document goes for 12 pages), are offering API allowing to send messages to that platform.

Any people with experience on that topic ? Pricing ?

Why you need to put WhatsApp messages in a web application when you can use WhatsApp via web.whatsapp official site.