Yes, be very careful if you use this mod(especially during installation). The replacement back piece that they give you is made from a much thinner glass than the stock iPhone case(because there needs to be room for the LED panel to sit on top of the battery). I have heard of several people breaking the glass during installation.
My glass is still in great condition, I use a bumper for my 4S. However, I am always especially careful with my phone.
You should try contacting K.O. Gadget to see if you can possibly buy just the glass, but I am sure the glass is the most expensive part anyway so it probably wont be any cheaper.