Well, here's some updated info from my original post.
1st, XM won't "automagically" renew me at the good rate, as it turns out. I have to call a day before the current arrangement expires, and go thru the "I can't afford it; give me the good deal or cancel it" argument all over again.

However, they seem to always have the good deals available, so I keep renewing at the "5 for $20" rate.
After my 90-day "freebee" trial of OnStar, I signed up for a year of the basic safety subscription and negotiated $50 off the standard $199 price. I didn't opt for the turn-by-turn feature, because it works like crap on my '06 XLR, and is a waste of the additional money (they want another $200 for it). However, when my wife's '11 SRX "freebee" trial expired, we signed her up for it and got like a "hunski" off the standard $399. The turn-by-turn works great on her car.
One aggravating thing that OnStar did was to change my phone number on the car without even telling me. I noticed the different number on the subscription paperwork, called them up and raised hell. Eventually they came back and told me that the "old" number (which I had given out to friends) was not available any more (Verizon had issued it to someone else when OnStar fumbled it). I gave them (politely) a piece of my mind and told them that they needed to do something to compensate me for the problems of having to contact those folks to whom I had given the number. They came back a few days later and "gave" me another 300 minutes of phone time and extended the expiration date. I'm OK w/ that and am pleasantly surprised that they cared enough to do it.
Hope this helps some of you in your negotiations. FWIW
Tim