The memory and speed limitations of voxels have been mostly overcome (at least from a game programming perspective) by advances in modern computing hardware. The problem now is that voxel objects look "chunky", especially when used as a first-person 3D engine. Imagine a scene rendered in 320x200 and displayed in 640x480 and you'll have a rough approximation of what current voxel first-person engines look like.

Or, you can go to and see some screenshots of Ken Silverman's in-progress voxel engine. They adequately demonstrate both the strengths and weaknesses of voxels.

This is the greatest challenge to overcome with voxels. There have been many suggestions, such as storing data in a voxel structure but then using polygons to represent the data on the screen; this would still allow marvelously deformable levels (imagine every rocket impact taking a chunk out of a level wall and eventually breaching it) while still looking acceptable, but would also require rendering many more polygons than a traditional polygonal engine would.