Don't confuse cryobiology with cryonics. Cryobiologists, the people who actually invent these tissue preservation techniques which are routinely used in hospitals and research labs all over the world, typically think that cryonics is a pseudoscience at best and a fraud at worst: http://en.wikipedia.org/wiki/Cryobiology#Scientific_societies

Reversible vitrification of individual cells or small samples of tissue is possible because they are small, thus they can be cooled quickly. Cryoprotectants are used to facilitate the process, but not in toxic concentrations.

Fast cooling of objects as large as a human body, or even a human head, is essentially impossible due to the square-cube law: the thermal capacity of an object is proportional to its mass, which, for a given density, is proportional to its volume, while its capacity to transfer heat is proportional to its surface area. As size increases, surface area grows quadratically while volume grows cubically, hence their ratio decreases.

If you attempt to cool a large object too fast, you will freeze or vitrify only a thin superficial layer, and probably even shatter it, since temperature gradients cause gradients of thermal contraction resulting in mechanical stress.

Cryonicists who attempt to preserve whole human cadavers or heads, perfuse them with large amounts of cryoprotectants in order to achieve vitrification. This has several problems:

• In contrast with mainstream tissue preservation techniques, cryonicists use cryoprotectants in toxic concentrations. At these concentrations, unreversible damage occurs: proteins become denaturated and cell membranes become distorted.
• Cryoprotectants are perfused post-mortem. It's unclear how deep they are actually able to diffuse. Any area where cryoprotectants don't reach the concentration required for vitrification will be destroyed by ice crystal formation. So far, no cryopreserved human brain has ever been examined to determine the extent of freezing damage.
• The cryoprotectant perfusion process and the subsequent cooling are very slow. Typically, at least two days pass between the someone's terminal cardiac arrest and the time they reach glass transition temperature, during much of this time their brain has no significant oxygen and glucose supply (ischemia). Human nervous tissue is typically unrecoverably damaged after about one hour of ischemia.
• For ease of storage, cryonicists cool cadavers past the glass transition temperature, down to liquid nitrogen temperature. Since different types of tissues in the human body thermally contract at different rates, mechanical stress causes multiple widespread macroscopic fractures in all organs including the brain. The extent of microscopic damage at the edges of these fractures is unknown.