I wanted to replace the existing 1TB HDD in my 2011 iMac with a new SSD one. Other World Computing rates this task as advanced, and I agree.
They had a video for me to follow that was pretty close to what I needed to do. In it, a new HDD was put into the same iMac - so armed with that video, and the video showing how to mount the SSD drive - I and a friend were able to install the SSD and get the computer closed up in about an hour.
But then our trouble really began. I could not install OSX on it. I tried a USB drive, older install DVDs etc, but I was never able to even format the new SSD.
In the end, we did a network install - where the OS was downloaded from Apple somewhere. This did work, and soon I was up and running. With the new SSD installed, there is no comparison with the older hard drive. The computer *feels* 10x faster - like new. In this sense, I feel this install was highly successful. It has definitely bought 2 years more life (at least) for this computer.
Suggestions: During purchasing, OWC should ask more questions about exactly what you're buying the system for, and include a sheet of paper with the components that has the URLs to the exact videos/web pages etc you need. Crucially, this should include how to install the OS on your new computer - as it was clearly dicey in my case.
The classification of the level difficulty (i.e., advanced) was about right. I've built several PC's before, worked as a programmer for several years. I think you'd want that level of experience before undertaking this upgrade - so I wouldn't recommend it to any of my friends. I would tell them to pay for this service.
Finally, OWCs marketing on how much faster your computer will be is not hyperbole. A newly installed OS on SSD is just so much faster. Even though it was hard, and I wouldn't recommend it to friends, I'm glad we did it.