Never used them in beer, but I have made wine with them. The wine, when properly aged (elderberries are very high in tannins) has a port like quality to it. Elderberries were once used to beef up port, before it was banned by the British, and Elderberry wine is occasionally refered to as "English Port", hence the Monty Python insult from the French.
One thing to be aware of is that you need to add them in the boil as they have a toxin in them that needs to be broken down with heat.