We just got our first rack with back heating. Here are some things to know:
1. For us, and others I have read about, the back heating simply could not keep up when the temperature of the room dropped below 75 degrees Fahrenheit. Now, I have my first space heater I have ever had to purchase simply for the snake room. Joy!
2. I do like the back heating because it gives a really nice temperature gradient. Instead of having heat directly over the belly flexwatt, you get a very hot back, radiating nicely to a dropoff in the front. I have to imagine that it's easier for my snakes to regulate their body temperature that way.
3. I think it's more energy efficient? Correct me if I'm wrong, but I think having a strip down the back would be more effective than having a strip at every level.
Just watch your ambient room temperatures, because you might find that the hot spot temperature will just keep falling and falling and never recover if the ambient temperature in the room gets below 75 degrees or so!