I have been using Whatsapp for about 18 months now.
Interface - It is well designed but after the new update of the version, I find the settings part a bit not good.
Use - It is best for connecting with loved ones within seconds, although there are many other apps but this being a popular one, is used the most.
Design - Design was also previously better than the latest update to the app.
Chats & backup - I was disappointed when I uninstalled and reinstalled whatsapp and my backups did not restore for no fault of mine. I lost all my chats which was very disappointing as I had some important messages.
Support - Support is also not good as when I contacted them, they responded very late and also their response was not helpful. They only responded what was mentioned on the FAQs in the website which I had already read. Being such a popular app, they need to up their support.