I have found that adjustable shoulder rests are a great thing to have. A lot of shoulder rests just stay one size, but an adjustable one can change to any way you want it to fit. Just saying, you can’t go wrong with one.
My favorite shoulder rest is the Everest. I like it because the feet are made of solid rubber. A lot of shoulder rest feet are plastic, covered with rubber tubing. When the tubing wears down, the plastic can damage your instrument. 🙁
The Everest is slightly adjustable, and I find that is all you need. You can make it wider/narrower, and adjust the height of the feet.
Kevin – Shoulder rests are not necessary to play, but they do offer a good amount of comfort and stability for players who are getting used to proper positioning and technique. You may notice some players, such as Itzak Perlman, do not use a shoulder rest. This is simply his preference, as you will also find other incredible players, such as Hilary Hahn, who swear by the Kun classic shoulder rest.