Valium is not used much today, because of its habit forming traits. Also in the older patient (over 60) it can have a cumulative effect since it is long acting and more slowly excreted by the kidneys. It is not really recommended for the elderly.

Benzodiazepines start off working well, then tolerance begins and they lose their effectiveness. This leads to increasing doses, and eventually the decision to discontinue...which is a very hard thing to do. Back when Valium was new and non-controlled, many people had nightmare things happen when they stopped using it. This movie dealt with the problem and is quite famous:
http://en.wikipedia.org/wiki/I%27m_D..._Fast_as_I_Can
I can recall back in those early days, we would go thru 5 or 6 bottles of 500 a week where I worked ..that is how popular it was. I was interviewed by a stringer for the NYTimes about Valium abuse
then, in fact. (I remained anon--didn't want our patients to see that in the paper). Since Valium there are other benzos, like Klonopin which are even more antiseizure, and harder to discontinue! Xanax is shorter acting and has become the "anxiety" drug of choice, but its problems are similar when used long term. Sudden discontinuance of a benzo used chronically can result in seizures in some people.

Some of your feeling better has to do with a more restorative sleep rather than the drug, I think.
It would be better to deal with the leg cramps another way, than to use Valium every night, IMO. An occasional tablet, which is not every day, usually is well tolerated however. It is when it becomes regular use, that is when you face a painful future.

I have found the muscle relaxer Flexeril very effective for muscle pain. I use it occasionally. Just recently when I had my root canal, and my neck gave me grief for a couple of days.