I have a Verizon cell phone, and I've added the additional line into the H2 through the OnStar system, so for ~$80/month I have my cell and the H2 with 700 minutes to share... I'm very happy with it. Sometimes OnStar is a biotch when trying to saying the numbers to dial (doesn't like the 717 area code for some reason).
I've also used the OnStar for remote diagnostics (more than a few times), to find Hummer dealerships (you know, when you got without directions), for roadside assistance (free towing to your nearest dealership, 5 gallons of gas, flat tire change, etc. for $12-$17 month), and I like knowing it's there if I have an emergency or accident. I don't think you can go wrong.
Plus, if you have someone who's wasted and won't stop pestering you to take the truck out for a spin, you can always threaten them with "I'll call OnStar and they'll find you if you take the keys!"
