We had a mid-2011 27" iMac that had never been used much.... sometimes bigger isn't always easier to use! Moving some furniture allowed us to consider using it again but it wouldn't start up, no lights, no sounds, just dead. Rather than recycling a barely used machine we decided to risk spending the money and replace the power supply.
The repair guide, and similar video demo, was very helpful. Things went smoothly with no surprises.
The only trick I would offer is using a short piece of a plastic straw to hold the T10 screws upright when reinstalling the screen. The magnets around the perimeter of the screen are very strong and can grab the screws making it difficult to get them in their holes straight. By using a 3/4" piece of plastic straw inserted directly over the hole, the screw will easily slide down straight making insertion simple. Once done the straw pulls out easily.