I have been playing Empire Total War for about 2 weeks now at a very intermittent pace. I an still working through the "Road to Independence" tutorial campaign but now that I have finally reached Chapter 3 most of the restrictions seen to be removed and it feels like a full game. Chapter 3 opened with my American rebel forces being overrun by the British redcoats at Bunker Hill which was a bit disheartening until I read that this is historically accurate.
I am enjoying Empire more than I expected having read reports of bugs and poor AI. I was initially concerned that the advent of powerful firearms would remove a lot of variety from the game because one soldier with a musket is much the same as any other. Having played a bit however I haven't had problems with bugs or AI and I find the changes to combat intriguing. There is something very satisfying about stopping an enemy charge dead in its tracks with a hail of crossfire from overlapping volleys.
Increased accuracy and damage means cannons now rule the battlefield capable of routing units before they even get into the fight.
I am still trying to figure out where cavalry fit in this new order of things. They have the advantage of mobility and flanking engaged infantry still works but the horsemen seem a lot squishier and suffer heavy losses. A frontal charge into gunfire is suicidal. Cavalry have an important role countering artillery and they can destroy cannon in seconds. If there is an infantry unit anywhere near the cannon though the cavalry probably won't make it out alive.
Edit: One other observation about the impact of fire-arms in Empire is that the balance of power has shifted firmly in favour of defenders. The shattering impact of volley fire from prepared defenders makes charging a very risky tactic.