"......Its suppose to expand twice its size and aerate the potting
soil...Is this an improvement over vermiculite? Is it expensive?.."

Available under many names around the world, and most will make a lot more
than twice their size when they have taken in water.
Very useful in tubs, hanging baskets etc as well as in ground that dries
rapidly, the use will reduce the number of waterings required as well as
lessening the stress on the plants.
In the ground should last several years.
It is also said to help drainage and to prevent waterloging in pots etc.

Have a look at http://www.soilmoist.com/soilland.html



--
David Hill
Abacus nurseries
www.abacus-nurseries.co.uk