My late 2012 MBP was showing the worse signs of aging -- constantly beach-balling, super slow response times, apps were slow to launch, start-up was interminably long, etc. I tried everything: cleaning programs, nuke and paving it (which helped for about a month) but it was always coming back to a machine that seemed older than 4 years.
I had always heard great things about OWC but despite being very savvy on the software side of Macs, I was nervous about how I would do with a hardware upgrade. I'm not a naturally handy guy. So I used the OWC site to see what I would need and I watched their excellent install videos to see what I was getting into -- I ordered a SSD and a memory upgrade and hoped for the best.
And it arrived and I took about a hour and half slowly moving through the process rewatching the OWC video over and over. Moving my old spinning hard drive to the the old optical drive port (which gave up the ghost in the MBP's first year), put the SSD where the old hard drive was, and upgrading the memory to 16GB.
Turned it on and WOW! it's an amazing machine now, even better than it was when it came out of the box in summer of 2012.
I really recommend it -- it makes working on my Mac again not only tolerable, but joyful and it was so much less expensive than what it would have cost me to get a new MacBook Pro or iMac. Again, don't let taking apart a computer intimidate you -- the videos and advice here are super solid and if I can do, I feel confident you can too. Really adds a whole new life to a machine I thought was more or less dead.