Rapier is a breaker program which breaks 1 firewall and has no cooldown. Its PWR cost is equal to the current alarm level plus 1. Rapier can be chosen as a starting program, or bought from some Server Terminals for 600 credits.
Rapier's PWR cost stops growing after alarm level 6, unlike the bonuses from Network Siphon and Reactive Myomer. If you buy Rapier part-way through a mission, then the effect of the current alarm level applies, although the in-game description does not make this clear.
At alarm level 0, Rapier hacks for 1 PWR per firewall, which is efficient compared to other instant breakers without cooldowns. At alarm level 1, Rapier costs 2 PWR to break one firewall, making it equivalent to Lockpick 1.0, and then at higher alarm levels it is worse.
So, for Rapier to be better than Lockpick overall, you have to do most of your hacking at alarm level 0. This is at most 5 turns at the start of the mission, so fast and early exploration is essential, and you should avoid raising the alarm tracker in other ways. At alarm level 2 (or alarm level 3 in Beginner mode), Rapier becomes much more inefficient also because the firewalls of unhacked devices are raised, so you should try to do almost all of your hacking before then.
Rapier works best in a loadout which generates PWR early; most generators are fine, but Dynamo and to a lesser extent Fusion are slower at the start of the mission. Internationale's Wireless Emitter works well with Rapier, since it lets you take PWR from Consoles sooner. Seed can be a good choice, since it still lets you use Rapier for 0 PWR once per turn at alarm level 3, but Seed does not replace the PWR you use up by hacking at alarm level 0, so you may find yourself very short of PWR later in the mission.
Rapier is at its worst when you urgently need to hack a Security Camera or a drone with high firewalls at a high alarm level. Even just hacking 3 firewalls at alarm level 4 costs a prohibitive 15 PWR. So when you play with Rapier as a starting program, you will want to buy a second breaker such as Dagger, Hammer or Wrench early in your run.