For simple and fast projects it is very easy to use. In my opinion, its best feature is its project library, where you can find all of your sketches.

Super simple to use! Sit down and learn the kit within a few days and then the tool is open to your imagination! I use it for basic prototyping of interfaces for product prototypes

Arduino makes things easy. Like, really easy. You don't need to know that you're compiling C and getting it through the Atmel toolchain; you just hit "upload" and select your serial port and it works. It's' fantastic for beginners, and can be used to teach programming and electronics to people with no experience. The built-in libraries are useful and allow you to easily use common components (servos, LCD screens) without learning the details of the protocol yourself.