I've had XM radio for over 5 years now and I've never paid full rate. I have subscriptions on 3 radios and go through the hassle every 6 months for each of my radios. Then they give you the speech about how this is a "one-time deal and then pricing will revert back to full price". I just tell them that's not true, I've been doing this for years
They can become a real pain to deal with and have even told me that the 6/$25 deal doesn't exist. But, I have some old radios that I deactivated a couple of years ago and I keep getting those deals in my email so I tell them I know it exists!
One other thing, many people are saying not to give your credit card info. Why not? I do it to make sure my service does not get interrupted. If you monitor your credit card activity, you'll be fine.
They usually bill me at full price after the "promotional period", but then I call them back up and go through the same deal. They will credit you back for the unused portion and you'll just end up paying full price for a day or two when you recognize you've been charged. One tip I can give, if you do this route and they credit you back the difference, you MUST request that they give the money on your card. Otherwise they'll keep it on your account and it'll be difficult to tell you've been charged full price next time because they'll use up that credit first, then bill your card.