valarray is kind of an orphan that was born in the wrong place at the wrong time. It's an attempt at optimization, fairly specifically for the machines that were used for heavy-duty math when it was written -- specifically, vector processors like the Crays.
For a vector processor, what you generally wanted to do was apply a single operation to an entire array, then apply the next operation to the entire array, and so on until you'd done everything you needed to do.
Unless you're dealing with fairly small arrays, however, that tends to work poorly with caching. On most modern machines, what you'd generally prefer (to the extent possible) would be to load part of the array, do all the operations on it you're going to, then move on to the next part of the array.
valarray is also supposed to eliminate any possibility of aliasing, which (at least theoretically) lets the compiler improve speed because it's more free to store values in registers. In reality, however, I'm not at all sure that any real implementation takes advantage of this to any significant degree. I suspect it's rather a chicken-and-egg sort of problem -- without compiler support it didn't become popular, and as long as it's not popular, nobody's going to go to the trouble of working on their compiler to support it.
There's also a bewildering (literally) array of ancillary classes to use with valarray. You get slice, slice_array, gslice and gslice_array to play with pieces of a valarray, and make it act like a multi-dimensional array. You also get mask_array to "mask" an operation (e.g. add items in x to y, but only at the positions where z is non-zero). To make more than trivial use of valarray, you have to learn a lot about these ancillary classes, some of which are pretty complex and none of which seems (at least to me) very well documented.
Bottom line: while it has moments of brilliance, and can do some things pretty neatly, there are also some very good reasons that it is (and will almost certainly remain) obscure.
Edit (eight years later, in 2017): Some of the preceding has become obsolete to at least some degree. For one example, Intel has implemented an optimized version of valarray for their compiler. It uses the Intel Integrated Performance Primitives (Intel IPP) to improve performance. Although the exact performance improvement undoubtedly varies, a quick test with simple code shows around a 2:1 improvement in speed, compared to identical code compiled with the "standard" implementation of valarray.
So, while I'm not entirely convinced that C++ programmers will be starting to use valarray in huge numbers, there are least some circumstances in which it can provide a speed improvement.
