I'm using my NavTraffic in Houston and it is great!
First day I had it I was coming back from the Woodlands around 10pm. Just as I got into downtown Houston the system gave me an alert on an accident that had the southbound interstate shutdown. I grabbed the next exit and routed around. Cars were backed up for 2 miles on that one, NavTraffic saved me hours, worth every penny...
I'm going month by month and it added $3.99 to my price, I think the total is right at $16 per month for XM and NavTraffic...