If a carry has on-hit effects, nab it.
Varus is the best example of who to get Runaans for, especially as blight amps the damage on his other abilities.
Ashe/Twitch are good examples also, the latter more so because of expunge. only problem is I don't think it works off spray and pray.
I wouldn't really get it on anyone else though.
Vayne can't use it since silver bolts only affects one target at a time.
It seems like an item more for on-hit builds than for ADC. Even in it's best case scenario it's a very niche item.
© 2013 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.