I don't even care if they hit anything. All enemy, no matter how stupid, mostly avoid their cone of fire. This is useful to me for defense. This is the main reason that I have them installed.

They don't hit things well, so bait (like a workbench) or water (make them swim) or similar tricks to get the enemy to stand still is needed for anything not already gigantic and slow. Conversely, agitated enemy run faster and are unhittable. Lighting fires under grey dwarves is not the way to hit them.