I had to replace the rear glass panel, as it was cracked.
It was relatively simple, but there were no written instructions included. I wouldn't consider myself to be handy, so I would imagine some other people would have trouble with the process. It all turned out just fine in the end, but I did scratch the paint on the glass panel thinking it was what was covering the adhesive.
Use plenty of heat. Apply the adhesive to the phone first and then place the glass on top. Make sure it is all flat so that the glass is flush with the edges of the phone.