To answer the question, no.
I might be interested in a "reasonable" 1 time fee for use of the remote app for the life of my vehicle. Or maybe if I can bundle my wife's acadia + my truck on 1 plan I'd consider it. But paying $15 a month per vehicle just to use the app?? can anyone who does this confirm if you have multiple GM vehicles, can you access both of them for $15 or is it per car? I'd pay about $5 per month total for 2 cars, for what I used the app for when I had it. So they are only 6x higher than my price point. I know you can bargain with them for like $9/month trial period, then they raise it up again.
We have signed up for a month of unlimited data a couple times when taking family trips in the past. But with current cell phone plans and where we drive, my wife just uses her phone as a hotspot now as it gets better service. Onstar is basically useless to me.
Go google Onstar complaints and read all the stories about it being impossible to cancel, they just keep charging you. Don't give them a credit card number.