I don't really listen to the extra content, because I have Sirius as well. The digital signal comes in and out, just depending on where you are. When it is digital, it sounds fine, but it's far from CD quality.
I wouldn't spend much extra to have it (again), and I wouldn't let it be a deciding factor in my choice of head units. Even though satellite radio has a monthly fee, it's a lot better.
If you don't go in expecting much, you'll be fine. You might find extra content that you really like. When it first came out, I really thought the sound was going to be amazingly better, but it's nothing special.