Metal Black

Although born as Gun Frontier 2, Metal Black feels much more like a sidequel to Darius. The gameplay however is dissimilar, and mainly stands on collecting orbs to power up a laser weapon, with no other weapon variations avaiable (although the more orbs you collect the more powerful standard fire is as well).
This laser weapon is put to devastating effect especially against the bosses, who have laser beams of their own. If both the player and the boss initiate a beam the effect is very similar to what you’d see in Dragonball, with a big sphere of energy slowly heading against the weaker of the two (depends on how charged the beam was) with the end result truly apocalyptic explosion. This of course is incredibly satisfying.
While not exactly “bullet-hell” it does present quite a bit of action on-screen, which is somewhat rare on horizontal shooters, which usually seem to favour narrow corridors and shifting-terrain to keep movement difficult. The Metal Black ship also feels quite heavy or slow, almost Invader-like, perhaps to give an idea of power.
There are 6 stages and a couple of bonus stages, although it is still a moderately difficult game. Unlike Darius the stages are sequential and cannot be chosen.

Graphically the game’s incredible not much by enemy design (again, mostly lifted from Darius), but especially due to the special effects in explosions and the fantastic backgrounds. Along with SNK’s Last Resort this might just be the most impressive looking shmup of that generation.
The music while not as interesting as in Darius Gaiden is still pretty good.

Playing through it I realised with surprise how much Invaders Infinity Gene ended up borrowing. Many of the enemies, the powerups and even the evolutionary design seem to have been lifted from this incredible game. More than an hommage to Invaders per se, Infinity Gene seems to be the evolutionary synthesis of Taito’s shoot em up genre history.
G-Darius and Border Down (G.Rev) also seem to have been favourably impressed with the game and have borrowed several elements as well, however I haven’t had the chance to delve into them.

Satakore have an incredible gallery avaiable, as well as a video of the "arcade-perfect" Saturn version.
The game is of course also avaiable on Taito Legends 2 (Xbox, PS2, Windows)