My early 2008 iMac was having hard drive problems. The HFS+ directory was becoming corrupt on a daily basis, but the drive showed no bad blocks and passed its SMART tests. Only the extended hardware test on the OS installation CD showed the problem: 4HDD/11/40000004:SATA(0,0).
There's nothing like taking apart an Intel iMac to make you appreciate the engineering that goes into them. There's not a lot of empty space in there and many delicate components block access to the hard drive. The guide is very helpful and the tools were good quality, though the bits on the driver kit are not labeled, so it make take some trial and error to find the right size.
The loop of tape around the display connector can be frustrating. In my case, it was necessary to pull the loop apart and pull on both ends to dislodge the connector.
Be mindful of the microphone cable as you screw the front bezel back on. It can easily block the nearby hole you need to put a screw into, forcing you to remove the bezel to reposition it.