My iMac was running super slow. Every test I ran on my hardware reported no issues. However I concluded the issue must be my HDD. So, I decided to slap in an SSD.
I have heard/read horror stories about the sealed up Macs of recent years and how they are terrible to work with. I was rather nervous to start, but I knew I wanted to purge my Mac of its dreaded HDD problem. I went ahead and purchased all the tools and parts as listed in the iMac 21.5 HDD replacement guide.
Right after opening my box from iFixit, which came super fast btw, I dove right in to fixing. I followed the guide step by step and was amazed at how easy the tools and guide made the whole process. I pulled of the screen, swapped my HDD for an SSD, replaced the screen with new sticky tape, performed a internet recovery, and BAM my iMac is as good as new. Actually its better than new.
All of the "Macs can't be worked on any more" stories I have heard sound a bit silly to me now.
Don't listen to the naysayers. If you need to fix something do your own research and try it for yourself. This can save you time and money. That sweet sweet sense of accomplishment once you are done is like nothing else.