I find America is a very easy civ to play with if you want to go on a conquering spree. I just build tons of ranged and siege units and sit them one hex out of site of the city/unit I am fighting and blast the oblivion out of my enemy, as the UA of America is increased range of sight. The UU's of America help with this too, the minuteman avoids terrain movement costs so it's really hard for it or any upgraded versions of it to get bogged down in a fight I don't want it to be in, and the B-17 is my pet city killer... seriously, I often just wait until I can build B-17s and then its game over since the B-17 has siege promotion. Also the extra range of sight UA means I can just go settling the countryside with unprotected settlers for the most part as the settlers will be able to see any threat coming in enough time to move out of the way, that is given the proper terrain is there (hills nearby, open plains, etc.).
What difficulty are you playing on? In my experience, once you hit Emperor(?)+ your enemies are going to have enough units that any unguarded siege or ranged units are going to be minced meat pre-artillery. I can see it being useful on lower difficulties, but even then I don't see how that makes them better than, say Siam, especially when compared with some not-that-great UUs.