Greek Fire is very much a tradeoff strategy, but it really comes into it's own around the third/fourth reform. That's when it gives a whopping +1 to infantry fire, later going up to +1.2 and a +20% infantry combat ability.

That said, I haven't followed the Janissary tree through past the second reform yet. Honestly, any military strategy is viable here, the choices seem to be: Logistical Army, with larger force limits, decreased costs, and eventually no professionalism loss for mercenaries; a Basic Professional army, with a discipline boost and some other bland bonuses, but no real weaknesses; a Cavalry army, with boosts to flanking range, cavalry combat ability (potentially stacking with Aristocratic Ideas and the Hippodrome), and cavalry/infantry ratio increases; and the two special chains: Greek Fire, which is heavy on the Fire phase of combat, with penalties to shock damage received, cavalry combat ability, and the cost and size of your army, and huge bonuses to infantry, and some minor bonuses to siege or artillery combat, alternately.

Update: Janissaries go away on the third reform. It is rather disappointing.