If you enjoy playing barbarian factions, and more generally if you enjoyed RTW overall, then yeah.... I think it's worth the money. The "hording" dynamic (where certain displaced factions can generate large armies) is an interesting addition that keeps things stirred up on the strategic level, and you'll have to pay attention to religion. I like the Sassanids, as an option for playing a civilized faction other than the Romans, with lots of interesting units. Also, BI finally forced me to learn how to use horse archers, which I hadn't really focused on before.

I liked BI, maybe you will too. But I think it really does depend on how satisfied you were with the core RTW game. There aren't any massive improvements in some of the things people complain about most, like the tactical AI or diplomacy.