Wanted a faster disk in my older 2009 27" iMac which otherwise works great. Prices and reliability on SSDs seemed to be pretty good these days, compared to replacing the entire iMac. I bought a Samsung 830 series SSD.
I followed the excellent iFixit guide "Intel iMac 27" EMC 2309 und 2374 Festplatte wechseln and had no difficulties. Overall, it took maybe 30 minutes. I used a Silverstone 3.5-to-2.5 bay adaptor, which worked perfectly.
This was easier than it looked -- the glass comes right off without much effort. The lack of a temp sensor in the SSD was easily fixed by shorting the cable with a small piece of wire -- don't be afraid of that hack!
The funny thing is that the day after installing the new disk, I got an email from Apple about problems with the 1TB disk, offering to replace it. I'll keep the SSD, thanks!