I had one of these installed in the spring: Viper - Viper 3303 Responder LC SuperCode 2-Way Security System
It's got the interior proximity sensor. When I had it installed, I had it adjusted for full doors, no top. There's a small box with 2 screws that adjusts the range of the interior proximity sensor. I had the installer show me how to adjust for no doors/no roof, and after a little trial and error in my driveway, found good settings for that configuration. I marked both settings on the box with a Sharpie.
With full doors on, you can walk up and look in the window before it gives the warning chirp. With rock doors on (or no doors) you have to stick a hand inside the Jeep before it'll chirp.
Mine's gone off once this summer because of wind, and that was from a 35+ mph gust that nailed me while I was walking into a Lowe's. Not sure if my Safari Straps have anything to do with it, but I've had no issues with birds or tree branches.
I use mine generally as an early warning system when I'm carrying camera gear in my Tuffy box. That's really the only reason I bought it.
If somebody wants your Jeep, they'll get it, alarm or not.