All units are generic in appearance, except for the one unique unit for each country. IIRC, the US get the navy SEAL - a kind of beefed up marine.

Contrary to some opinions in this thread, I am still having a blast with Civ4 and highly recommend it. It is just a much more competitive single player experience than the Total War games - the AI really bites at my heels even at Prince level and there are times when I think I am done for (curiously, these are often when there are several stacks of Romans heading my way).

In terms of war fighting, Romans can steamroller other factions in the ancient period (their unique unit is Praetorians). My faction of preference is the English and although I try to play peacefully, their redcoat unit is extremely powerful in the gunpowder period (I am currently fending off an attack by French grenadiers and it is like Waterloo all over again).