I haven't got the links to hand but there exists voxel engines that put most poly rasterizing engines to shame. As Silverwarrior said, it's only a matter of decreasing the voxel resolution etc
Voxel models are volumetric by definition. So a voxel model doesn't need textures applied as the voxels themselves make up the 'pixels' of the texture. Whenever you've seen a CT scan of a brain, a 3D volumetric volume, that's rendered using voxels (or is ideally rendered as voxels) For example, a game that used voxels would let you cut an orange in half, see the segments seeds etc
(note this is totally different from destructable walls and things inside modern PhysX games, the texture data isn't volumetric, it's all a uniform 'wall' texture wrapped around the bits of wall)
With A poly engine, the inside of all geometry is empty space.
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The main reason we're not seeing lots of voxel engines is we're still one generation away from cards that are powerful enough for a voxel engine to match the quality of a current generation engine.
Another reason is Skinnning (you may know it as Skeletal animation, bones etc) as with a poly model you only have to skin the vertices in the model, a small amount of data. A Voxel model can't be skinned easily. You'd have to apply the result of the current pose to every single voxel. A vast amount of processing in comparison.
Until cards are powerful enough to skin per voxel or until some clever workaround is found, we will not see mainstream voxel engines do EVERYTHING with voxels. But we will defintely see static scenes, maps etc rendered using voxels in PC games, perhaps as soon as 2 years from now.
For rendering in 3D, radiosity and all manner of high end 3D techniques, voxels are vastly preferable to polygons. In the hypothetical situation where a voxel and poly map could be rendered using the same amount of work, more advanced things such as raytracing, radiosity could be applied far quicker and more simply in the voxel engine. When a ray hits a poly and you want to know the colour, you have to calculate the texture coordinates from the face you hit then sample the colour. A ray hits a voxel, you've already got that information (although I see an issue here, the poly face will give you the surface normal, so unless every surface voxel stores it's normal, you'd have to sample surrounding voxels to determine the angles... but John Carmack said it's better and he's a lot, lot better than me)
And another big advantage is Level of Detail. in a Poly engine, you control the textures LOD (and with more recent tesselation hardware) the poly LOD seperatley.
A voxel engine has no textures as such (the colour values of the voxels in the model equating textures) and the LOD calculations are oct-tree based, so you instantly have a really simple way to handle LOD that works for everything in the same pass.
Voxel engines are a far more 'realistic' simulation of reality than poly engines. It's a poor example but a voxel engine would let you for example heat something up with a blowtorch until it was white hot, then you could watch the heat dissapate thru the model in a natural way. Or destructable terrain or blowing holes in enemies etc are ridicuously simple in a voxel engine but require some complex maths to perform boolean operations in a poly engine (Red faction anybody?)
Voxel engines are the future, don't take my word for it, take John Carmacks.
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