There is a thread on the ATS forum about OnStar service.
I added my $0.02 to the discussion a couple of days ago describing my experience with OnStar customer service last year... pretty horrible!
My actual experience with OnStar while I had it was that it sent me to wrong location twice and actually got me lost once with sending the wrong route to my Nav system.
In my wife's SRX, I use a Garmin Nuvi 2595 that is amazing for POIs and connects to the phone via bluetooth ('07 SRX does not have bluetooth). A few months of OnStar fees will pay for the GPS. As well, I have free lifetime updates for the Garmin.
All that being said, (no app for me for 2010 model in Canada) I suppose the plusses for OnStar are:
a) can locate your car if stolen and disable it
b) can send help if you have an accident and are unconscious (and nobody is around)
c) you can make a phone call if you are in a remote area with no cell service
d) can unlock your car if you've left the key fob in the car and its battery dies (just don't leave your cell phone in the car at the same time!)
Unfortunately, it can really ruin your focus and concentration while on a road course when an operator calls to see if you are OK because they are seeing your vehicle generating unusually high G-forces!