Vermiculite, although it has its drawbacks, can be very effective as for gardening if mixed with another type of medium. A very common mix is 50% perlite and 50% vermiculite. This works well because the perlite is very porous and doesn't retain moisture while vermiculite can retain 200-300% of its weight in water.
Dangers of Using Vermiculite
The major drawback of growing with vermiculite is that it is very dangerous when dry and may contain small amounts of asbestos. It can become airborne if touched even while wet because it will dry on the fingers and turn back into dust. This also makes it very difficult to reuse because of the rinsing and drying required.
Another problem with using vermiculite is its water retention. If used by itself (not mixed with perlite or another medium), it may strangle the roots and kill your plant.