Superweapons give you an edge. In a slower game, it is a large edge. In a faster game, not so much. That said...
Deliverence engine:15% extra firepower wherever you fire them at. You can also barrage someone's homeworld (with about 3 of them) dropping not only homeworld loyalty, but that of all planets that get more than minimum 25% or whatever it is. While this can eventually cause everything to decolonize, it more importantly hits their economy pretty hard. You can also leave this superweapon on auto fire and it just ... gives you free loyalty wherever you fire it.
Novalith Cannon:1-3 cannons can uncolonize a turn and eliminate all the population and thus taxes. Oh, it can be recolonized quickly, but the tax takes a lot longer to get back. Decolonizing a planet also suspends any attempts at rebuilding static defenses or shipbuilding. In fact, about 2 of them can probably permanently render a shipyard system largely inactive.
Kotsura cannon. Ships are stopped for 15 seconds, orbital structures become inactive for 3 minutes!!!! yes, that means you fire a cannon so that it hits just as your fleet is arriving, half your fleet will spend 15 seconds shooting their inactive fleet to hell, THEN your whole fleet gets another 2 minutes, 45 seconds to shred their static defenses. It also means if you know where their shipyards are, you can disable them and keep them that way. It also means if your fleet is 2 jumps away, you can fire them 15 seconds apart
None of this has "take over a system" stamped on the side next to the "easy button". They are not C&C superweapons and if you do not know HOW to use them, they will seem weak. They are just strong enough that they can allow a faction with one less level of max cap, or to drain your opponent's resources. Without shields, you build 4, that recolonization (need to get a colony ship to the right place) lost income while the pop recovers, and lost orbital build time (can't build/rebuild orbital structures if there is no colony) is something the other side has to pay TWICE every what, 3 minutes? The development cost will pay itself off within 15-20 minutes most likely, and that's if you build 4. If you build 8... well it's a big investment, but it pays itself off, even IF you do not use the weakened state of the colony to help your invasion push.
Kotsura basically renders any and all orbital defenses (except fighters) moot. Note that you can have a carrier fleet and just bombard repeatedly while your fighters take out their fighters and whatever else you want.
Deliverence engine:It is a subtler weapon, but quite nasty. You hit a planet with it, they can not colonize it for several minutes and you get a 15% damage bonus. It can block enemy culture. It can supress enemy culture. It can be used to ALLOW you to colonize a planet that would otherwise have too much enemy culture. Basically, it means you can get all the major bonuses of culture and disengage all the disadvantages in a small area (per engine).
Are superweapons worth making a mad dash for? Not really. Are they worth researching at some point in many games? Yes.
The same can be said for the later improvements to trade ports and refineries.
THESE SUPERWEAPONS SHOULD NOT BE SEEN AS 'BIG GUNS'. THEY ARE AN INVASIVE INFRASTRUCTURE ADVANTAGE!!!!!!!!! They do not and SHOULD not replace fleets.