I made a few trips to Houston, Texas with my 9 month old. I pretty much stayed in the back with her - she slept most of the way, but when she was awake - I make sure I had her favorite toys to play with. We read lots of books and played "peek-a-boo". I also brought along some of her favorite songs on a CD for her to listen to. I did learn the hard way, not to let her drink a lot while we were driving (it gave her bad tummy aches and she cried for about and hour until I figured out what was wrong!). So we stop over hour and half to two hours to have a drink if we need to, and get her out of the car seat and walk around a bit.
Since were going to visit family - I did learn that after a 10 hour car ride, it took her about a day to recover where she would let people hold her.
But other than a little gas every now and then - taveling with her when she was little was great. Now that she is two - I would dare attempt a trip longer than two hours without a movie for to watch or books for her to read! She is way to restless now!!!