Played with the Callidus last night. We were were in a teams battle - 3000 pts against 3000 pts. I wanted to test my zealot list out on vets night.

On my side were doublewing ( termies/bikes ), vanilla marines w/ drop pods and my zealot force - each weighing in at 1000.
Our opponents were guard ( with alot of tanks ) and night lords ( not too fluffy but meh - on vet's night you play what you bring).

The wh performed well. My penitent engine and arcos drew fire for two rounds away from the rest of the forces and one arc actually hit the lines but rolled a six on the attack. My storm trooper squads took out two infiltrating guard squads despite sustaining heavy losses from tank bombardment.
The zealots marched up table but never reached the opponents tank line as the game was conceded after four hours! at the end of turn fiver turns - barely in our favour. It was a cleanse gamma mission. But the Callidus! On turn two it jumped in and flamed two havoc squads - placed way to close together - and then attacked one consolidated into the second and held off a imperial guard squad that decided to join in the mellee and after losing combat fell off the table edge. Not bad for one assassin. This not even taking into account a squad of snipers left in the open ( word in your ear ) to be shot up by the Inq and retinue on turn one.
Deathwing was on their game; taking out a Chaos lord, two squads of furies, Lascannon squads and two oblits.
Zealots got some kills with their two grenade launchers, the assasin was eventually overwhelmed by numbers after taking out two havoc squads. I forgot to jump back on turn two or maybe I could have templated twice.

I would add FAGK to my list as allies next game. The Arcos did little as the terminators dominated the board making inv after inv and taking down tanks and infantry ( the SM also drop in some termies to back up his melta drop pod tac squads.

Lessons learned - Callidus was awesome and swung the game turn two by taking on two havoc squads and greatly reducing our opponents fire base. I also learned that GK would be better then Arcos ( ha ha). Now penitent engines are also a lot more durable then given credit for as they ignore stunned and shaken and draw a lot of fire power. I used to use the Eversor and really liked him but after this game I'm a convert - the Callidus simply does what needs doing. The flame template seems almost like cheating and jump back is good although the game was conceded before I got to template twice. I also forgot to divide my Arcos into two squads which would have helped.
It was nice teaming up with all those terminators, I must say. I also think that tank heavy IG has way too many weaknesses. Drop podding termies anmd meltas along wioth the whirlwinds really hurt the IG force. The NLs were left almost to fend off all our forces on their own in the end.

My list was ...
Inq w/ retinue of warrior hb, chirgeon, penitent
Priest form zealots
6 flagellants
2 squads of stormies w/ 2 plasma each
Squad of twenty zealots w/ 2 grenade launchers 3 evicerators
Penitent engine

I want to drop the arcos for a chimera and some FAGK next time around.