Um I think it is worth it. I have it and live in VEGAS. Its like "LA Lite" out here in many ways, including traffic. If the road is red from traffic, I would say most of the time when I get to that part, traffic is indeed at a standstill. In addition to highways, it also appears to show major city streets. I have seen it update even as I arrived at a standstill area.
I know they offer gas prices, weather and movie times also. That stuff I do not find as useful. For one it takes forever to pull up. For two I have an Iphone and well, see part 1. I can get any of that crap in about 10 seconds on my phone, takes sometimes (no bs) 10 minutes to pull up on the NAV. Neither one is especially easy to use for that purpose while driving so that's a tie.
Nonetheless it is cheap. I think its like $80 for the whole year or something. So if you do a lot of driving it is worth it. However, maybe I just keep the map zoomed in too close but, I find I am alerted to traffic issues fairly well by these freeway signs we have here that basically are for traffic alerts.
The two work together well though. Example = today. Driving home the alert sign over the road said "road blocked one lane open" or something after my exit. Sirius showed traffic flowing ok except right before my exit. I would say between the two it perfectly described what I actually encountered, heavy traffic before my exit but nothing that slowed me down too much.
Don't know how much more detail I can give. For the money it is decent. Says on their web page its just $3.99 a month for the traffic only. I'd do that if you're cheap as I find the other parts of travel link useless if you have a smartphone.