Who? Reznik? He's an odd mix of anti-magic and brute force.
I think you have to be a crafty player to use him well because his base size makes him easily exposed. However, he does have a few advantages over the Feoras (though maybe not against them due to Immunity Fire).
1. Reach. Even though he's slower, Reach gives him the ability to attack past models in the way if you can draw LOS to the target in melee. Feora1 can only attack in .5" However, Feora 2 has both Reach AND a self teleport spell, but she loses Engine of Destruction.
2. Witch Hound. What's every eHaley's favorite spell? Telekinesis. What about Aiyana and Holt? Harm (or whatever it's called now). How about pDeneghra? Crippling Grasp. In Menoth you can create a dilemma with Reznik and Choir. Make it no spells or make it no shooty and count on the enemy to not want to cast spells. Of course, making use of Witch Hound means your enemy is forgetting or thinking the trade is worth it.
3. Movement. eFeora gives her Warjacks extra movement. pFeora turns them into auto-hitting infernos for a cost of 4 foc. But Reznik cancels out one of Menoth's glaring weaknesses. He can grant Pathfinder to a 'jack. I think only Amon can do this, but that's Amon. In addition, the 'jack with Iron Aggression on it gets boosted melee attack rolls. The problem with the tactic is that they can see it coming. Who cares? Maybe they'll cast a spell at that 'jack
In addition, Perdition gives him an offensive move his warjack around option. Not the worst thing in the world to have.
4. Anti-magic: Purgation on himself is amazing, but it's also dangerous. He suddenly can get boosted P+S 18 attacks against a target with an upkeep spell on it. Be careful, he'll be exposed at that moment, especially if you cast Engine of Destruction to get P+S 18 attacks (equivalent to a 'jack, in case you're wondering where that falls on the power scale). If you do hang out Reznik to dry (and it will be easy to because he can charge 10" and reach out to 12") you have to use his feat.
Judgment Day strips fury and focus and prevents focus allocation and upkeep spells. If they're running 'jacks, they'll have to rely on the warcaster to finish off Reznik, and they'll have to re-cast any useful upkeep spell that they want to use against him, lighting them on fire.
If you're playing against Hordes, however, the warlock will be starved, but the 'beasts will be able to force. So be careful.
Hopefully, that's a helpful response.
What are your thoughts on High Executioner Reznik?