Freaking awesome. I have never played a more interesting game except for its predecessors.
There are a few things that can be troublesome. Disease will strike your army at the most inopportune times like when you're raiding a village or beseiging a castle. If you don't have a doctor or fast access to a place where they can rest (and even when they do rest you're bound to lose some) you may lose your entire host. Depending on the circumstances, you could be losing hundreds of thousands of thaler (that's the currency in the game). Another thing that can really piss you off is getting shot from across the map. Armour is fairly useless when facing a horde of gunmen so you might as well choose some clothing that you like.
All that said, you'd be stupid to not pick this game up for just $9.99 on Steam. The open-endedness is not by any means a drawback, but the thing that keeps you so immersed in this world.