This talent creates Healthstones which heal on use. Before patch 3.0.2, a healthstone with a different number of talent points spent into this talent was a different unique object: it was possible to have three healthstones on hand rather than a single one. This was often used in long fights such as Onyxia so that the main tank could have three healthstones of the highest rank.
All ranks of Healthstones, improved or not, are on the same cooldown timer.
Patch 1.6.0 (12-Jul-2005): The increased effect from this talent should now properly apply to other players that use the Warlock's Healthstones.