I've used one (in training). Works great, nice clean cuts, but still a little slow. Quicker to just use an axe which, if you're a firefighter, you have handy. (Carefully chop around the perimeter of the windshield or side window. Use gentle blows to minimize dispersion of broken glass within the cabin.)

Okay now I'm imagining a classical ballet recreation of the "Heeere's Johnny" scene. With firemen.