I don't have much experience with the others, but I can give some insight on how to make a starpact warlock work at levels 3-4'ish.
You pretty much have to have Improved Fate of the Void. Astral Fire is also very helpful.
Your first few turns are spent cursing something that's on a flank, using Dire Radiance on it, and Shadow Walking to the other flank where you can curse something else next turn. Since whatever you hit with Dire Radiance likely needs to move closer, you'll be doing more damage to it. If it doesn't move, then you've successfully done a bit of board control. Second turn, you do the same on the other flank. Third turn, you may get to do it again or not, depending on positioning. You may just be better off with an Eldritch Blast. Position yourself so you can curse at least a third target.
By this time, your allies should start dropping your curse targets, and with a +2 or +3 bonus, you may have the attention of your Cleric as a good bonus stacking target. Your allies will also typically have used a few encounter powers and will be running low. It's your time to turn on the burner. Use Frigid Darkness on something tough, to help make the best of the few remaining encounter powers your allies have, and for your next attack.
If the encounter is challenging (you'll have a good idea by this time), use Diabolic Grasp to slide something away so it has to move closer to you or an ally next turn to attack, and follow it up with an action point to toss Dire Radiance. Now the target will also take damage when it moves. And both these attacks are quite likely to hit thanks to Frigid Darkness from the round before.
Then you fall back on Eldritch Blasts since you will prefer to target Reflex over Fortitude for the most part. If you have gained any temporary hit points, try to spend them by walking right up to your Fighter's or Paladin's mark and Eldritch Blasting them. This will provoke an opportunity attack from the enemy, which in turn will either provoke a combat challenge attack from the fighter, or do Divine Challenge damage, while you are benefitting from Prime Shot. Thanks to your good Con, you can take a few hits, and have a decent number of surges so you shouldn't have to worry too much about spending those after combat.
Try to find a Rod of Corruption, as early as you can. If you can curse a minion and kill it early on in a fight, you'll be collecting pact boon bonuses fast.
Clearly, it's not as easy as, ho hum, I use Twin Strike to shoot my quarry twice, done, next. But a Star Pact Warlock is still quite fun and fairly effective.