I've only ever used the Mega Gargant once and to be fair to my opponent they were really unprepared for it! I usually go for a mass horde Ork army so as you can imagine their army list was full of AP weapons with some AT to deal with Battle Wagonz etc.
The Mega Gargant alongside a Big Supa Stompa detachment (1650 points) destroyed about half their army (1500 points) in the first two turns!
For the rest of the game though they were rather out of it as they had stayed on my base line to deal with his planet falling troops while the rest of the army had advanced.
Overall, I thought they worked really well but could probably be avoided due to their slow speed if the opponent had a a lot more activations - I only had 4 activations in the entire army.
I think they would work quite nicely against other large Titans as they have fairly high amount of TK weapons.
Not exactly what you were after but its the only experience I've had with them!
