The main problem with voxels is the large amount of data which they imply. This is because voxels are normally only considered to be 3D bitmaps. A small 512 x 512 x 512 contains over 100 million voxels (a lot of data). To put this in perspective here is a 2D 512 x 512 bitmap:
That's about 4 times the size of a single face of a rubix cube. So, if you wanted to make a voxel landscape for example, you would potentially need many gigabytes of memory (That is of course, without any sort of compression).
By rethinking voxels as models rather than images, I think I can reduce the memory requirements to be little bigger than that of an image.