I went later (Feb 12 - Mar 13) because I didn't want to deal with the shorter days. I too went W-E, but finished in Pensacola.
Using El Paso as an example:
1. In December the day length gets as short as about 10 hours and 4 minutes
2. Starting Feb 12th the days are already over 11 hours and by my finish date of Mar 12 were over 11 hours and 30 minutes.
So at least consider that if you have the option of going later.
The weather was mostly nice, but with frost some nights and one night that got down to 18F. The 18F night was when I was on a pass overnight so it could have probably been avoided.
Camping was plentiful enough and in Texas, which is a full third of the route, I just camped in roadside picnic areas much of the time and even under a bridge once.
I didn't think traffic was bad. I did ride either the interstate or US 90 a good bit more than the AC route used.
I found the people nice, the food good, and the scenery pretty boring.
My trip journal is at http://www.crazyguyonabike.com/doc/ScrapingBottom
Oh and I found it pretty easy to average 77 miles per day on that route.