The piano is a wonderful first instrument to learn. Unlike other instruments, "no assembly is required," nor does it need to be tuned each time it is played. A beginning student can immediately focus on learning rhythm, notation, and fingering without having to wrestle with pitch and tone.
The piano is also a wonderful second... or third or fourth... instrument to learn! The pattern of black and white keys on the keyboard provides a tactile and visual platform for many music theory concepts. The instruction in music theory, harmony, ear training and classical repertoire contributes toward a comprehensive music education.
Playing the piano builds physical coordination and enhances fine motor skills. Pianists learn to read notes on the grand staff (both bass and treble clefs). Music for piano is often self-accompanied; that is, the pianist provides both the melody and the harmony. There are many mental and physical benefits to studying the piano, but the emotional benefit is invaluable. The ability to read and play music is a joy, and sharing music with others is a gift.
One drawback - the piano is not as portable as most other instruments. An average upright piano weighs 300-400, pounds so this is not an instrument to accompany you on a backpacking trip in Europe!