Yeah I've built it. Finding the hammers isn't too bad once you stop and think whether you actually need whatever else you are focusing on. It is one of the few bonuses in the game that is an investment rather than a quick pay-off. Whether that investment is worth it is up to personal opinion I suppose.
I like to combine it with the 15% from Honor. Double open terrain + HE + Honor + GG = 75% increased combat strength. Turns long swords into rifles, rifles into infantry, etc. If you can get a couple of melee upgraded to march, they are nearly invincible.
The flip-side argument is, of course, that a bomber stack is just as invincible with little to no focus on promotions. Although at the very least it is an option to consider if you want a game to let samurai or berserkers really shine.