I have one, and love it. Range is at least 30 yards, I can start my truck from the living room without problem. If the key isn't put in the ignition within 12 minutes, the engine cuts off automatically. If the brake pedal is depressed without the key in the ignition, the engine cuts off, so it seems pretty fool proof to me. When I park during the winter, I leave the defroster set, and it really makes clearing the windows a breeze the next morning. My keychain control also unlocks the doors, so I can hit it from about 15 feet away when walking to my car, and not have to pause to manually unlock the door. Handy when my hands are full, and a nice safety measure for my wife in parking lots. The doors lock automatically when the igntion is started, and unlock automatically when I cut off the ignition.(These things can be customized to your preference when it's installed) All in all, pretty handy. I got mine at Best Buy about 2 years ago, for $150, under the name of 'Auto Valet' or some such. I like it enough to put one in my future trucks as well.
They work in most all modern vehicles with automatic transmissions, so I expect they'll work in yours without problem. Best Buy will install it for free if you'd like.
If you get the hummer, drop me a line and let me know what you think of it.