From a defensive stand point I think they are pretty overpowered.
From a offensive standpoint they seem quite good but having to wait a turn to set up could be a significant drawback if enemy ranged units can fire over your front lines, dmging the siege unit before it gets a shot off. This could force the Siege into retreat early in offensive battles, forcing you to bring multiple siege units to reap the benefit of there power.
From a cost to power standpoint siege units are much more efficient than archers however. But they require a resource to make, so that equals things back out a bit.