Finally got one and installed it last week. So far, I'm happy. I had the 2nd previous model to this one in my last truck. When I ordered my LML I opted out from the factory nav as they are junk (IMO) and I like the flexibility of the aftermarket units.
Yes, I had a Alpine in my last one, I like it, just not as well as the kenwood. The Garmin Nav is also a big plus for me. I never had a Pioneer though.
The PAC OS-4 adapter takes care of that. It allows you to still use onstar and make calls from it. All it does is kill power to the radio and "switch" to the onstar module and it performs just like factory.