It's extremely powerful. You typically don't need anymore tech and when you do:Could do it but that seems terrible. And dangerous, although in some instances you actually never need a new tech again.
The massive surge in units nets you tons and tons of land and new cities much quicker which as we know translates to more happiness resources, reseaarch, production, etc. Old cities eventually get out of unhappy faces (cause you're researching again). If you're CRE throwing down a few cheap inexpensive buildings like Theatre, Collosium, and owning dye, etc lets those cities become productive much quicker.
My favorite unit would probably be the HA, follwed by the Catapult. Even on Deity it's too ezy to quickly whip 30+ units in 10-12 turns in the mid BCs and rampage. While cannons and rifles are good my games don't typically last that long.