Something not mentioned in any of these writeups regarding the Stellar Converter is that it is an active, usable weapon during space combat; you aren't limited to "just" destroying planets with it. A single shot can turn a planet into an asteroid belt. Think about that for a second. Now aim the bastard at an enemy ship. Heh. About the only class of vessel that can stand that kind of pounding is an equally-equipped doom star.
With a "properly" equipped doom star class ship in MOO2, a single ship can take on a planet and its defenses entirely on its own. With defensive systems that a) make it hard to hit, b) reflect and/or absorb damage taken (reducing or eliminating damage done when a hit is actually scored), and c) converts damage into energy, you get a ship that's difficult to destroy. With offensive systems like the Stellar Converter, not one of your enemy's opposing ships will survive even one round of combat when the beam is fired. I have seen the AI build precisely two ships in all the games of MOO2 I've played that could withstand a blast. Shields down, "we're leaking ship guts down here, captain!", good luck getting out of orbit ... critically damaged, in other words.
Did I mention the Stellar Converter never misses a shot? No jamming, no cloaking, no real defense; if your ship can see the enemy ship and has the Stellar Converter on board to fire, and you give the order to fire, that enemy ship is about to absorb a full blast of planet busting death, at 100% strength, with no modifiers, even if your defenses include them. Oh, and it's on a 360-degree mount, too, so it can fire in all directions without loss of accuracy.
The Guardian? Fucked. Opposing fleets? Fucked. The Antarans -- hell, even these guys are fucked.
My ship designs always included the Stellar Converter once it's been researched; hulls that can barely hold the thing get at least one design like that -- it's a space-faring death hulk with nothing on its mind but the pumping of raw death juice into the carcasses of its opponents. Once you get fairly high up the tech tree, though, you'll never build a doom star without a Stellar Convertor. Granted, you'll spend the rest of the ship's space with defense and repair systems, but hey -- one big gun works here.
Combine this weapon with the advanced terraforming abilities (whereby you can shape even an asteroid belt into an appropriate planet for your use) and you get instant expansion powers -- waste the enemy fleet, vaporize their planet, and rebuild it to suit your needs. Pure evil. But it's the most entertaining thing you'll do to a computerized (or even a human) opponent in any strategy game, ever.