My 2011 21.5" iMac came standard with a 500GB hard drive, and after 8 years, we were out of storage, and probably running out of time. If the typical life of a hard drive is around 6 years, then we had been rolling the dice for a while. I had previously added RAM to my iMac, but that was a minimally invasive install.
I have to admit, I was intimidated at the thought of cracking open my iMac to install this new drive, but the instructional video was incredibly clear and accurate. Every intricate detail is well explained with close up video shown of exactly how to execute each step. The included installation kit came with everything you need, except for a needle-nose plier to carefully retrieve screws, but most people who would take this on will have a pair of those around. Even determined as I was to take my time, I think this only took me about 25 minutes, start to finish.
I haven't noticed a huge difference in the speed of my iMac, but most of our files are saved to external hard drives, so it's limited by the connection between the drive and the iMac, which is USB 2.0 due to the age of the machine. Mostly I like the thought that my new Seagate hybrid solid state drive will keep our iMac running for many more years, and that the 2TB of storage has put a stop to the warnings that I'm running out of space.
I didn't even bother to price out what this kind of upgrade would have cost installed through the Apple Store or a third party computer service. The price offered by OWC and MacSales.com was the best I found anywhere, and I saved a big chunk of change doing the install myself.