I agree with most of the comments above so I wont add my own. Redguard's is pretty spot on.
My own real negative so far gameplay wise is after conquering several cities in Britian am now up against Picts and Irish with nordic units....que? It is a bit of a let down. A few more unique celtic units could have spices things up. I'd have also perhaps liked to see another Irish faction and perhaps a few more cities. An extra in Ireland and another in Wales....(no welsh faction?).
I like the fact playing as the Roman Empires you get the "testudo" which seems to work. Managed to hold off two stacks of Visigoths with just a few units and only lost when my general fled due to archers (yes I should have charged but though they were gonna run out of arrows).