It was super easy, at least in my case. I use Google apps for everything (including Google Photos) and 99% of my texting is via WhatsApp.
I just followed the on-screen instructions and allowed it to wirelessly transfer everything over. It even brings over your logins/passwords. I backed up my WhatsApp chats, but it turned out I didn't have to as it all showed up on the new phone as well after the transfer.
Obviously make backups where appropriate in case something goes wrong, but it's extremely straightforward and it basically clones your old phone into the new one. Settings, app locations on the home screen, and even WiFi passwords etc. are all intact and just how you left them.
It asks for the SIM during the setup process and I just installed it when it asked me to.
The nice thing is you don't have to do anything to your old phone until you are satisfied that everything has been moved over to the new phone, so you always have that backup as well and you never have to take a 'leap of faith' for lack of a better term haha.