Repeating Om is probably useful in the same way all meditative methods can be useful. It is important that any meditative method not be practiced mechanically. What I consider more important is the understanding of what meditation is and what Om represents. The closer we get to the correct understanding the more effective these things become. In the end it is the understanding that is more important than the practice. With enough understanding there is no need to do anything. Memory is a recording of the past that is given importance because it is easy once a memory is created all we have to do is refer to it in any situation occurring in the present. We have become addicted to this way of living that does not require effort or attention this is why meditation is difficult. In meditation we can no longer rely on memory although we tend to continue to do so. Meditation and ultimately understanding require that we learn again how to live in the moment with full attention. This requires an understanding of the right effort needed to achieve this natural level of attention. If the past is memory what then is the present? Clearly the present is the place where a choice may be made as to how the potential inherent in the present may be used or created. In the moment we can either choose the past in the form of converting the present into the past with memory or not. This 'not' is important as by not converting the present into the past we remain in the present and in the present we have access to creation. All meditation should lead to the understanding of this potential of the present which is where the universe is created. We are interfaced with creation itself but somehow have become lost in a world created with memory that can never be as fresh and creative as the present. With memory we create a world for ourselves that is distorted, corrupted, and untrue and then wonder why we suffer. Suffering itself is a memory that we choose over the present just because it is easier to do this than to do the work that is initially required to find our way back to the present. It is not just specific memories that are the problem it is the fact that we impose our memories over everything to the extent that all that we see, feel, or understand about ourselves and the world is a bad recording of the past.