For many years, my colleagues and I scoffed at the Unity engine in favor of Torque because Torque gave us source access.
The physics engine was giving us crap? Code dive, debug, fix it.
Oh, one of their undocumented API calls is failing/you don't know what it does? Just look at the source and figure it out.
Granted Unity has a lot more documentation and a great deal of support, you still don't GET the source unless you pay quite a hefty fee. And it's still bugs me to this very day (Even though we've since switched to Unity)