Including the sun in your compositions can add impact and punch to your images. (Warning, to avoid damaging your vision, only look at the sun through your camera when it is very low in the sky). If the sun is a major part of your composition you will need to be sure not to stop down the lens down beyond f5.6, or else the shutter blades will distort the roundness of the sun in the final image. Flare can be a definite problem when shooting directly into the sun. Flare is caused by light shining into the lens and causing highlights on the internal elements of your lens, these highlights appear as aperture shaped highlights in your image. When shooting directly into the sun try to shade the front element of your lens to help cut down flare. Or line up your subject to block the sun and greatly reduce flare. Often times you can see the flare in the viewfinder prior to making the image. You could also embrace the flare and use at as a compositional element in your photo.
Lining up these shots can be frustrating I am always surprised at just how fast the sun is actually tracking across the sky when I try to make images like this. I find it easier to figure out the direction the sun is tracking, setup the shot just ahead of where it will soon be, and let the sun move into my composition.