First of all, this is one of the most beautiful cards in all of Netrunner. Second of all, this ICE is pretty good, but is competing with other HB code gates

Pros:

Brutal utility subroutines: Click Account Siphon is very powerful for the corp, especially in the crucial early game. A full fire means the corp can FA a 4/2 on their turn- choice targets could be a Vitruvius with a counter, or a Corporate Sales Team.

Value, even at last click- Enigma and Hourglass suffer from ignorability if the runner is on their last click-even if the runner has none to lose Nightdancer will still give you clicks

Taxing- 4 to break with Gordian Blade, Abagnale, Inversificator, and GS Striker M1 is respectable

High impact subroutines make Nightdancer a good target for a Batty game, as well as a good choice for Seidr Laboratories: Destiny Defined decks

Rare non-bioroid HB ICE, will surprise runners who think they are safe running at 4 clicks

Cons

Impact comes at a high cost- 6 is pricy for a 4 strength ICE

Weak to Cyber-Cypher and Black Orchestra (more evidence Black Orchestra is a mistake!)

Overall Nightdancer is an interesting alternative to the popular Fairchild suite!

Interesting ice because unlike most other ice its bite is not in setting back the runner but advancing the corp. Losing a is annoying but expected against HB. Giving the corp 2 extra clicks in a turn, provided the corp has some credits, is a massive risk of moving the game 1/3 of the way toward a corp win (4/2 score). Despite being utterly porous this is ice that a runner, outside of hail-mary runs, has to break.

A 6 4 strength code gate is a bit on the expensive side. For comparison Fairchild 3.0 is 6 for 5 strength (and one extra sub). That makes it a bit hard to justify putting in Nightdancer unless you have some backup plan to make sure the subs fire, such as Marcus Batty, or if you have a way to reduce cost like Jinja City Grid (from the same set). Note that because Nighdancer is not a bioroid the usual HB tricks for rezzing ice (Awakening Center, Bioroid Efficiency Research, Howler won't work.