My daughter dropped her (our old) iPhone 3GS and shattered the front glass. Her phone needed frequent charging so while I was repairing it, I decided to go ahead and get a new battery also. Took the opportunity to go ahead and order a new 4S rear panel for my wife's phone that was also cracked from a different drop.
The 4S rear panel is a snap! Anyone that can use a screwdriver can easily replace one in about 5 minutes.
Removal of the battery from the back of the 3GS case is a bit tricky, but it's not my first rodeo, so that went fine without incident. Replacement of the front panel took some time and patience, but it looks and works great. Thanks iFixit!
Go slowly when removing the battery. Highly recommend using a hairdryer to loosen the adhesive to minimize the chance of bending the old battery.
Replacing just the front panel on the 3GS is not for the faint of heart. Separating the glass from the frame was tricky and it was challenging to keep from bending the frame. Watch out for glass shards! Also, be diligent to remove all the glue residue to ensure a tight fit. Carefully align the two pieces before pressing them together as the adhesive strips do their job well. Slow and steady is the way to win this race.