Pros: Upright spears never jam. When an enemy dies atop it, then the body will show up in the building's inventory when the spikes are raised, but this doesn't affect the power at all. They can also equip menacing spikes. As is the rule of thumb, trap components are more dangerous than handheld, namely due to their size. Menacing spikes are a better weapon component, but cannot be used by the military, only used in traps. You can also equip 10 of them, which means 10 strikes in tandem, which can do a lot of damage real fast. They also tend to keep the enemy intact, and do not destroy equipment, so you end up with whole goblin corpses, for fewer haul jobs as compared to serrated disks, which will shred goblins like a sponge in a cheese grater.
Cons: It must be manually activated. I prefer to use a lever located somewhere safe, with the "pull lever" on repeat, and as long as someone is idle, the lever gets pulled frequently and pretty uniformly. Pressure plates are nice, but if the plates are within the trap corridor, that means that someone will be safe from the spears, because someone's walking over the plates, but also means that you won't need a lever puller. They can also get expensive, as a good 10 weapons can consume a lot of time and resources.
As for actual use, you want the enemy in the killzone as long as possible, preferably using selective bait animals and unlocking/relocking doors, or simply using two drawbridges to close up both ends of the hallway, locking them in the trap hall. As long as the enemy is on the spikes, any material will eventually kill, since it's just a matter of time. Since most goblin ambushers wear leather or cloth, copper would technically be enough to shred them, though steel will rip them apart in a hurry. Armored foes will require a tougher metal, naturally. I'm also not sure on this tactic, but it stands to reason that if an armored foe is distracted by, say, archers pelting it with bone bolts, then he'll get backstabbed by the spikes and the armor value decreases because the spikes gain the surprise advantage. Just be aware that your forces cannot safely walk over or fight in the spike pit while active, because spikes make no judgment on targets.