I've already used this product once (along with the iPhone 4 liberation kit), and then I dropped my phone for a SECOND time, breaking the rear glass. I immediately returned to iFixit to replace the rear glass.
It's extremely easy to fix, but you need a pentalobe driver (or the liberation kit).
If you have a 4S, Make sure you order the 4S glass and NOT the 4. However even if you do screw up (as I did) the iFixit folks are really cool about helping you get the right part.