They lie somewhere between melee and ranged weapons. They aren't good for fighting off enemies from multiple angles or single, fast-moving enemies. They are devastating against enemies with many segments or tightly-packed clusters of enemies, such as the Eater of Worlds.
As of patch 1.0.6, if you hold down the left mouse button the flail will now stay out and not return to you. And, will continue to damage enemies while it is out. Also, the size of the flail's spiked ball has been reduced. As of version 1.1.1, flails can no longer be reforged with any size modifiers, ruling out Legendary and leaving the best prefix as "Godly".
There are currently five flails.