We haven't even scratched the surface of what this expansion offers, and I am already digging it. For starters it added a bunch of roles to the game and updated one old one. The majority of the new roles appear to have a mobility enhancing ability - this lets you get around the board faster which is a big help. There are also a few new special cards to help give the players some hope.
The meat and potatoes of the expansion though, is to make the game harder. There are new Epidemic cards that you can add. These will trigger an Epidemic, but will also do something extra - one of the cards can bring back cubes of a color that you have eradicated. There is also a new mutation challenge, it looks like about half way through the game a new color of cubes pop up.
The most interesting thing in this expansion to me is the Bio-Terrorist scenario. We haven't tried it yet, but it kind of looks like a game of 'Where in the World is Carmen San Diego' - how cool is that?
Overall this is a real solid expansion. It doesn't chagne the nature of the game - I am looking at you Settlers of Catan expansions - but adds a lot to the game and gives you the option of playing it differently.