I was rather shocked to find out that, if you hit at least 5 models with 3 suits armed with flamers and give them plasma and a multi-tracker, they do more cost-effective damage than Fireknife.
Assuming there's no cover save and 3 suits armed identically, on average you should get this:
Fireknife (Plasma/Missile/Multi) - 186 points
GEQ - 5 wounds, about 25 points
MEQ - 3.33 wounds, about 53 points
Orks - 5 wounds, about 30 points
TEQ - 2.09 wounds, about 84 points
T6, 3+ - 2.17 wounds, variable points
T3, 4+ - 5 wounds, about 55 points
Assuming you hit at least 5 models each with 3 identically armed suits on average you should get this:
Hot Knife (Flamer/Plasma/Multi) - 162 points
GEQ - 13.17 wounds, about 80 points
MEQ - 5 wounds, about 80 points
Orks - 13.17 wounds, about 80 points
TEQ - 2.92 wounds, about 116 points
T6, 3+ - 2.67 wounds, variable points
T3, 4+ - 8 wounds, about 85 points
Fireknife can only kill 3.33 Marines a turn in rapid fire range; Flamer+Plasma kill 5. Even against Marines, this is better for the most part turn for turn (of course the Fireknives have more range though). I've used this set-up with a Commander and 2 Shield Drones. They are surprisingly very effective against many kinds of enemies. They even survived a few games to the end, taking on multiple units.
Flamer + Fusion is very risky, but cheap and flexible and I've used that effectively as well.
As for deploying them, as others have said Deep Striking them behind a unit, notably the Pathfinder's Devilfish, and running them behind cover in some way if you don't want to play the risk. I do; I set them around 7" away from my target unit, sometimes 5-6" if I'm feeling lucky.
Another key thing is to charge after you shoot not jump away. Unless you can manage to get completely behind difficult or impassible terrain, you are going to get assaulted anyway. Your best chance is go on ahead and move into combat, denying your opponent that +1 A for charging and giving yourself +1. If it's getting hit hard by flamers anyway, you should have a decent chance with your suits against what's left.