I feel badly that they have gradually gotten less useful, as I'm generally a fan of them in general, and personally think that they are a fairly dwarf-appropriate approach to the problem. So when I'm doing a more elaborate fort, I try to find excuses to have them as part of the defenses, even if they're not cost-effective.
One such design I've been thinking about for a while is inspired by an upgrade to the "Bridge of Kazhad-dum" type entrance. Instead of a single long, narrow span over a chasm, have a square zigzag of such bridges (between tiny pillar islands). Then place at least one ballista firing down each leg of the bridges, but set back into the chasm walls, such that invaders never get close enough to terrify the operators. There can be additional, wider, retracting bridges that both short-cut the zigzag, and are not trapped (etc.), to provide for caravan access; the final one being a raising bridge to block the caravan entrance hole entirely, to prevent ranged invaders from getting lucky shots, and to force the invaders to continue pathing through whatever backup death-traps you have in the subsequent hallway. To me, this seems as if it would be visually impressive, suitably dwarfy, and has a decent chance of seriously cutting down the numbers of any invading force. Optional upgrades would include smoothing the walls of the chasm as you dig it, to reduce threat from climbers; and a magma-safe high-density chasm floor (for maximum impact damage) with optional and reversable magma pumping action, to dramatically reduce hauling and dwarven dead body exposure, and pre-process goblinite.