voxels are the meathod of choice for medical (radiology for example) and scientific purposes, because you pretty much need a supercomputer to make it work. they use it in those fields because its usually a method to display a mapped out scan of something by taking multiple samples of a 3d object, and using various color channels to indicate measured properties about that sample in the scan, like density for example. voxels are really the only way to display such data. imagine a 3d bitmap where in addition to horizontal and vertical resolution you have z resolution as well, this would make the image huge, this is also the typical output of, say, an mri scan. how do you convert that int a 3d model, it would be very difficult and use very complicated algorithms. so it ens up costing less to buy a super high end workstation to display the data as voxels.
i personally think that polygonal systems are far superior for gaming since you really only need to see the surface of an object and not necessarily see what its insides are made out of. voxels also apear to have gaps in surfaces or make things look to angular. and there are ways to bake additional detail into polygonal data to increase the detail, like normal maps. furthermore i just dont find voxel graphics to be very good. everything seems to be flat in color and the lighting just looks bad.