I have a 550, had been using XM for years, ST and Wing prior to my C14. I was only using it for tunes, but they gave me a free month of the weather and traffic to try. I have to say, for me anyway, that it was pretty useless. It would pop up a storm warning in my area, but I would already know about it in advance since I check weather on my Droid before heading home. The traffic was a bit better, but then, Denver isn't one of there listed metro areas... sucks. I usually have FM radio going on my Droid and pickup traffic updates while I'm on the road.
After all these years of XM service, I dropped it and sold my GXM30 antenna... run my Droid thru my 550. I'll listen to FM or Slacker, depending on my mood or if I'm traveling local or LD.