I just had my first tantrum spiral. Well, I wouldn't call it a spiral really, since the fort survived largely intact, but I've never had so many dwarves tantrum at once.
My fortress, Palebell, had preliminary defenses up. The walls were complete, and bridges done and linked to levers. However, I had no traps and my military was unarmed and had no armor. Thus, I had nothing in the way of early-warning systems, and a small squad of goblin macemen were able to sneak into the fortress.
They started to wreak havoc once spotted, and all I could do was send my 16 or so wrestlers at them. I had numbers on my side as well as a swarm of biting dogs and human caravan guards, but the goblins were able to take out several of my soldiers and severely wound many others. After all was said and done, a dozen or so dwarves were dead outright, a handful of them died of their wounds soon after, and many dwarves came into my hastily-constructed hospital with varying degrees of injury.
It took me a while to get all the dead buried, having no coffins built before the attack. My dwarves also took the loss of their friends and loved ones quite hard. Several dwarves tantrumed -- some furniture was destroyed, a few fights happened, and another few dwarves went insane, but eventually the fortress calmed down. I had about 90 dwarves before the attack, and about 70 when the tantrums stopped.
I erected cage traps outside the bridges in order to provide some defense against goblins able to get that close. Magma forges have just been completed, so I should have my dwarves kitted out in steel before the first siege. At the very least, I can seal the fortress off and shoot at them if they come before I am prepared.