One of the really great things about the Arduino design is that it is open - all the circuits and board designs are available for anyone to use. One key upshot of this is that once you have done some experiments with something like an Arduino Uno or Mega, and decided on how you want your circuit idea to work, you can easily re-create just the bits you need, in whatever shape suits your project. There are lots of people who have built their own "Arduino" on
hand-soldered stripboard or even on a
solderless breadboard. Adding extra components and your own wiring is just as easy, and there are plenty of tools to help you design printed circuit boards too.
In short, Ardiuno is not only a fun platform for learning about electronics and programming, but also scales smoothly into real products, if your idea is good enough!