I personally do not have a manastone yet - but I know some clerics that do.
In a group if you are with a shaman with regen, have them cast it on you -
before you use manastone - and it is best to use manastone if you are
already at full health, during downtime when you are medding between fights.
The clerics I have seen use the only lose about half health - and get so
much more mana... and remember, you ARE a cleric, if your health goes down
using a manastone, heal yourself :) The amount of mana spent casting
greater/superior heal is nothing compared to what you would have gained
through the stone.

Sirensa :)