Ground allspice has a spicy, earthy taste that is slightly sweet. It is quite strong and should be used sparingly (at least at first).
If you need ground allspice, just by a cheap coffee grinder (Krups makes one) and use it as your on-the-fly spice grinder. Ten seconds in the grinder and you will have fresh ground allspice. Always buy and store your spices in whole form if possible. They keep much long this way. Then just grind when you need them.