Well now that we are dissing Arduino, allow me to add my gripes!
Yes, very good, but the design is good enough that they are selling tens of thousands. The good points are that it is aimed at the beginner market, the shields are a useful add-on, and it was an excellent decision to use the open-source g++ compiler.
Compare that to other boards where you have to buy their own (dodgy) compilers, or get a cut-down "beginner" version, and put up with an IDE that is so complex it's almost impossible to get a project up quickly.