Took this photo of a robber fly perched on a blueberry bush and feasting on a honey bee. This is the only one I've seen. The university of Florida has a pretty good info page on these insects. There is a report of hundreds of these flies attacking a hive in a watermelon field. I didn't know they came this far north (MN).
http://entnemdept.ufl.edu/creatures/misc/flies/bee_killers.htm

http://entnemdept.ufl.edu/creatures/misc/flies/bee_killers.htm