From what I know Fioricet is the only drug that has a barb in it that is still prescibed fairly often. Soma works pretty well for me personally, but Valium is a better muscle relaxer (again, for me personally - I have some very degraded discs in my back and sometimes get very bad muscle spasms). And you have to remember, all of the drugs we are talking about here are pretty much 'older'. I think Valium came on the market in the 60's, it's known to be safe. But each person is different, that's why it's nice that as newer drugs come to market the older ones are available if someone responds to those drugs better.
The problem with barbs is that as your tolerance to their effects rises, your tolerance to a deadly overdose does not rise as quickly. So people will take a higher and higher dose to get the same effect, but eventually they'll hit a dose that kills them. On the other hand, a drug like Valium would be very difficult to overdose on, almost impossible (I'm talking about by itself, once you mix other downers like opiates, alcohol, then saftey goes out the window). I believe barbituates basically activate a GABA receptor, benzos just open them longer to your bodies natural GABA... somehow that ends up being much safter for you.
One of the interesting things about Soma is that it's not a controlled substance in most states, yet it's active metabolite is a controlled substance. Seeing that it's not a controlled substance it's actually very easy to find online and get 100% legally.