Love NaviGo, we have been staying in Ko Samui for a few months, and it has been the most convenient and best value transport we have found.
You will love it if you are an Uber or Grab user. Your order is via the NaviGo app, and you can pay cash or by credit card; you pop in the pickup and drop-off location to make it easy on the drivers.
You never have to wait too long, and if there is a language barrier, they have the details on the app to deliver you where you need to go efficiently.
Install the app if coming to Samui; much cheaper than taxis.
The cars are modern, clean and comfortable; I highly recommend NaviGo.