I had to replace a dead hard drive
It wasn't bad at all. The hard drive that was sent was not the same in the iMac. This means I could not use the thermal sensor. However, I found a website that allowed me to use it. Besides having big fingers, it wasn't that bad.
Have another person helping you. When you have to lift the screen, you have to be careful not to tear cables, especially the vertical sync ribbon. Also make sure the hard drive is the exact model that was already in there or else the thermal sensor is going to be tricky. Use tweezers for small cables if you have fat fingers.