I find the mobile app easy to use, although I can think of a few improvements. The website is also easy to use, and has benefits over the app such as being able to see potential returns before placing the bet.
The downside to this company is their customer service. On contacting them, I have found them to be rude, stubborn and not willing to help in any way. They did not want to listen to my views at all, just stuck to their 'read our long terms and conditions which isnt visible on the mobile app you use' stance. I work in customer services for an internet based company and even if our customer's issue is something stated in the terms and conditions, we still listen to the customer, sympathise and do our best to find a resolution. Paddy Power customer service do none of these things, and the appaulingly negative attitude of their staff has quite put me off using them again.