While it is convenient to send money to another person using WhatsApp but I have used it only few times in more than a year or so having it, even though on an avg' I do atleast couple of transactions a day.
One of the big reason for me to use it so less is perhaps because among my contacts and with whom I have need to exchange money were not part of beta program, BUT STILL I think P2P transactions are overrated as a focus area.
Most of my transaction happens due to ecommerce (food delivery, online shopping and others such), offline buys (ex. grocery store payment using shopkeeper's QR Code), transactions using bank app (Ex. for big ticket items like monthly rents, regular payments like electricity, credit card bills), and things like quick bank balance check which is super convenient using Payment Apps like GooglePay and others.
WhatsApp payment experience is very bare bones, it just focuses on casual transactions among ones contacts - not on deep and broad payment experience in different domains as one encounters in real life.
Famous last words perhaps: WhatsApp payment feature would be yet another payment mechanism in an already crowded space, it would not make much difference to us user's life, UNLESS they focus on payment space very deeply and bring merchants, online commerce etc. into picture.