Worked like a charm to upgrade Late 2012 21.5" iMac to SSD!
My Late 2012 21.5" iMac had been performing like a dog lately - the 5400rpm laptop hard drive in there is terribly slow. We didn't get the Fusion Drive model. I had upgraded Macbook Pro's to SSD's before, so I figured why not upgrade the iMac.
The repair was actually easier than first expected. It took well under an hour (even with removing all of the old adhesive and double checking on the new adhesive before installing it. The hard drive swap itself is a piece of cake once the screen is removed.
It is very helpful to have a pair of suction cups with handles (though not essential) to help pull the screen off. Have patience and work the cutter tool around the screen a few times.
The opening guides are right to stress to go slowly near the iSight webcam - that's the only place where you're in any danger of accidentally doing some damage.
The old adhesive will probably be dry in places and will feel like it doesn't want to come off cleanly. Just look for a good hold on the adhesive strips somewhere near the middle of each section - eventually you'll grab a piece you can tug on and the whole section of adhesive will come off.