The quad antenna is without doubt the best DX antenna available. It has a higher gain than any other antenna of similar dimensions and is also very cheap to construct. One of the greatest advantages of the quad is that it exhibits a lower angle of radiation than a yagi at heights of less than one wavelength. This makes it a great antenna for working skip as more of the power is radiated at low angles even if you cant get them mounted as high as you would like.
One of the reasons for the effectiveness of the quad is the fact that it not only exhibits more forward gain but also it has a larger physical capture area than a yagi of the same boom length. Each element is a full wavelength in size compared to a yagi which has elements of only one half wavelength making the quad a far more effective antenna for both TX and RX.
As with any antenna the true performance is only achieved when all elements are resonant at their correct frequencies and also when the feed point is matched correctly. I learned this from bitter experience over the years and I encourage you to take the time to do it right if you decide on building a quad.