The sign indicates the true personality, but not necessarily how the native appears or acts. If a person operates mainly through their superficial ego they will appear to be more like their rising sign (and its associated aspects, etc.) than their sun sign. If they operate mainly on instinctual needs or emotions, they will appear more like their Moon. If they operate mainly through their love nature they will exhibit the characteristics of their Venus sign, and so on.
Sun sign astrology
Sun sign astrology involves using just the sun and its placement and transits to determine behavior and events. For reasons already stated this is not accurate in predictions because people rarely act 100% like their sun sign. In addition, sun sign astrology does not take into consideration aspects from other planets which modify the expression of the sun, the real birth houses (which give insight into where and when events and experiences take place), and the individual's level of awareness (the more aware a person is the more they act based on conscious choice instead of instinct).
However, the Sun is a very powerful influence on the individual, and the sun sign is easily determined without the need for time and place of birth. Sun sign astrology provides a way to give a very general indication of an individual's personality and experiences. The Sun represents original intention, and therefore a study of the symbolic Sun in our lives gives insight into what we truly want to be. As long as one keeps in mind that the Sun does not represent the actual behavior and circumstances of the individual, study of the sun sign is a very useful application of natal astrology.
The study of sun signs could be improved by taking into consideration the condition of the sign's ruler and its dispositor. Each sign is ruled by a particular planet and the condition of that planet affects the nature of the sign. For example, the sign of Taurus is ruled by Venus. If Venus resides in Aries in the birth chart, this indicates the nature of comfort and security (Taurus) being expressed assertively (Aries). If Venus resides in Scorpio it more accurately represents the nature of comfort and security (Taurus) being expressed in a subtle and provocative manner (Scorpio). By at least considering the sign's ruler, a sun sign interpretaton could be made slightly more accurate, however it will still fail to accurately depict personality for most people.
Considering just the sign placement of a particular sign's ruling planet results in twelve different sign/ruler expressions for each sign of the zodiac (other than Leo). If we were to also consider the house of the sun's sign ruler we would get approximately 144 possible ways for Sun in a particular sign to be expressed. Then there are the aspects to the Sun and the aspects to the Sun's ruler. Combine this with the fact that individuals often express their Moon or Ascendant more than their Sun and it quickly becomes obvious that the sun sign alone is not a reliable indicator of personality.