The ease of use of the UI, plugins etc., is fantastic. The ability to easily add new functionality is also great. I have not found anything that I dislike about this product. Jenkins is easy for anyone with basic knowledge of scripting languages to get started with. It can be very frustrating if you do not have any programming experience, as there are many different features and plugins available.
If you do not know how to write scripts in Java or Python you will struggle, but there are lots of