Well, 256x256x64 isn't much, if you want a comparison its like having a cube with a 256x256 pixel texture. And while its easy to load a texture once and then use it to render every object that uses this texture, this becomes a bit harder with Voxels if you still want to maintain speed.
With Voxels its easy to interchange speed with memory usage, so if you have a few terrabytes free you can create stunning scenes with lots of details which renders blazing fast, but I guess your customers will hate you for it ^^