¶ Summary
While the Nightmare only has 4 turret slots, its flat 150% damage bonus makes its lasers deal as much damage as 10 unbonused turrets, without any skill training and at less than half the capacitor cost.
The Nightmare's main drawbacks are its sheer cost in ISK, and its demanding SP requirements. It is commonly used in PvE combat in Incursions, and in PvP combat by wealthy soloists and when well-resourced but outnumbered groups need to escalate to a powerful sub-capital fleet and cannot resort to the "n+1" tactic of a larger HAC gang. The Nightmare also sees frequent use by richer nullsec alliances as a fast, long-ranged, hard-hitting doctrine ship.
¶ Skills
While in most contexts Nightmares almost universally fit afterburners, an Incursion Nightmare is typically MWD-fit, so its pilot only needs the first rank of Caldari Battleship.
¶ Tactics
Since the afterburner speed bonus (150% with Caldari Battleship V trained) makes a Nightmare able to treat an afterburner much like a MWD on any other ship, it has a particular niche as one of the most powerful and tactically-flexible ships which can enter an ESS grid, where MWDs do not work and capital ships cannot go.