I have an iMac (from back in 2011) and discovered that the hard drive was on its way out. Looking around, I had a couple of options, replace the iMac itself (not really cost effective), send it to an approved Apple fixer (v expensive) or fix it myself. The problem was, the hard drive itself has been discontinued, and from the UK, expensive to buy like for like. I managed to find a place in Australia who was selling them for around £40 (around $65), so decided to plump for a FixIt kit - after all, I work in IT - so why not. A couple of clicks later from ebay and from ifixit - both parts were being dispatched from different points on the globe
Surprisingly easy. I had taken the week off work to do a few bits and bobs, and set aside a whole day to fix the iMac. The girlfriend and I were due to go out on the Friday night for a meal, and between work, run and getting changed, and with the iMac not working at all, I thought I would make a start. Within 20 minutes, I had the front of the iMac off, the screen out, the hard drive replaced, and cables inserted again. All that was required was fixing the screen and front cover back on, which I done the next morning. This was a bit of a bonus as I had the full of my week from work to play xbox, relax and spend eons restoring the data back onto the replaced hard drive.
What I did find, was that because I run alot of gaming on my iMac, I had a utility running to speed the fans up - once I replaced the drive and fitted it all together, the fans were spinning at the same speed as before, without the ability to switch the speed back down to "normal". I thought at this point that I had replaced a cable the wrong way, so had to pull it all apart and start from scratch and do the checks. Turns out I hadn't, so became a bit of an expert at it.
Make sure you have enough light in the room, and maybe use your phone light / torch. I also used masking tape to ensure that the cables went back in the same way.
Other than that - follow the instructions & you won't go wrong. I'm pondering getting the kit to add a SDD kit in, maybe I'll need a couple of days for that.