I know that they have a different level of acidity that they prefer in the soil. I have never tested my soil, nor added anything other than fresh layers of Miracle Gro soil and mulch over the fall leaves that naturally cover the ground. But every year, they bloom. They may not always be quite as spectacular as these photographs, but they are still beautiful. They compliment the bulbs that are finishing up and put color in the corners before being overwhelmed by the daisies and coneflowers.
Their cousin is the rhododendron. Those also have beautiful flowers, but they grow way too large for my taste. What I like about the azaleas is to be able to have similar flowers on a much smaller and much more manageable scale.