The hard drive failed on my wife's iMac.
I had never opened an iMac before. I followed step-by-step guide and was done in less than 30 min.
1. Have the correct tools handy - the right screwdrivers and definitely the suction cups.
2. Have some double-sided tape to re-attach pieces you'll remove from the original hard drive.
3. Have a can of compressed air to clean dust from the machine.
4. This might just be me, but when it came to removing the "bezel" I thought it was a thin piece. It's actually the entire aluminum front of the iMac.
5. You can definitely do this without detaching the LCD if you have a helper that will prop it up for the 5 min. it will take you to swap the drive.
6. Make sure to have lint-fee cloth to clean LCD before placing glass back on.