An already excellent game just got better!
Consequently, playing Rome itself is challenging, although you start off with a number of regions already under your control. However, you can also play as one of the Barbarian (hence the title) nations. Some, like the Franks, start with only a single region and have to work their way up from there. Others, like the Huns, take advantage of the new "Horde" system. If a Horde nation loses its last city, their people will form a huge army (Usually about 4 full stacks) to find a new place to live. The huns start out like this, which makes it possible to sweep down into italy in a few turns and settle your warriors there.
Another new feature is religion, which affects public order and the units you can build. Your generals are also religious, and if a Christian army is commanded by a heathen general, this lowers morale.
All in all, Barbarian Invasion is well worth the buy, as it introduces many units while removing some of the more tedious elements of gameplay (no Phalanxes, so no more 40-Minute-pushing battles). It provides a lot of entertainment for its price, and the new features, like Religion, are wonderfully integrated. So what are you waiting for? Lead your horde to Victory!