"No effect" is such a cold analysis and overlooks the benefits to the individuals praying, and to those being prayed for if they know it.
If anyone believes that Prayer will have a physical, tangible effect on some problem then they have another think coming. For those who are well aware that it is up to them to solve their own problems, though, then prayer can be a useful way of calming yourself, or focusing your thoughts. Also, knowing that someone will pray for you means, effectively, that they are thinking of you, and that they care. That's worth knowing, at times.