My late 2009 iMac was slow enough that I was considering a replacement, but after doing some research I decided to install an SSD and see what happened. I originally planned to replace the hard disk until I saw this kit from MacSales that would allow me to leave the original disk in place and put the SSD in the optical drive bay. I followed the videos on the MacSales site and completed the hardware upgrade in under 90 minutes.
I downloaded the latest Mac OS and installed it on the SSD. I ran into some trouble with the setup, and hopefully this review will help some of you to avoid the same trap. I skipped the Migration Assistant step during the OS setup, and this turned out to be a mistake. I used the Apple Migration Assistant later to move applications, files and settings to the SSD, and after the migration, the new user on the SSD could not access the files on the old disk because the new user did not have the proper permissions. I tried changing the permissions based on instructions I found on the Apple support site, but the procedure did not work. In the end, I erased the SSD again and started over. The second time I did the Migration Assistant steps during the OS setup, and this avoided the problem. The users on the SSD now have access to the files on the old disk.
As others have said, this upgrade has made a huge performance improvement for the iMac. Apps like Photos used to take 3+ minutes to open, and now they open in a few seconds. It was well worth the money and time involved, and it is not difficult if you simply follow the instructions in the video.
I also upgraded the memory in the machine from 4GB to 12 GB, but I believe the SSD made a bigger difference in performance.