For Sirius, the app is free but you need to have a subscription to the satellite service already or pay a small monthly fee to access the service.
And I was geared up to buy it when it came out recently. I was ready to hand over my credit card details for the $2.99 service for NFL coverage come fall. But no. Sirius seriously doesn't offer its NFL coverage to its iPhone app. Same with Nascar, MLB and even Howard Stern.
I can't recommend buying it for a service fee without access to the premium channels. Not with the plethora of offerings that don't involve a monthly fee.