I can tell you from past experience, be very careful with them.
I had five radios on my Sirius/XM account, at one time. I was catching the specials every six months and I would always call in, before my current promotional rate expired.
They will try their best to stick it to you, with those full-price rates. When you call in to cancel, they will put you on hold over and over again. Also they will tell you one thing and do another, so use the one-time CC.
Also, for those with the unlimited 'trial" subscription, I think that they send a "Kill" signal out intermittently and the radio has to be on to receive it. I have had them stay on for years with no sub, then suddenly die. Of course there is the possibility that your ESN was left off of their "kill" list.
For those interested in their hardware;
When I would call in to cancel, they would offer me all kinds of free radios, boomboxes etc. They told me that it's easy for them to give away hardware. I would then sell the stuff on ebay, after playing around with it.