Hey everyone, I've used Mac for awhile now and every time I get one, I replace the hard drive with an SSD.  I have used a hard drive version so I know what to expect.  The problem with this iMac was its super slow responses.  Anytime the drive needed to be accessed I'd have long wait times and the spinning wheel.  It almost looked like the app crashed but it was just slow.  Even opening up System Preferences was agonizingly long.  I've read people with this Mac had the same problems as me...but then other people said theirs was just fine.When I ran Blackmagic's Disk Speed Test, I was getting 25-30MB/s.  I ran this after upgrading to Catalina...I don't know if Catalina messed things up and made it slower.  My 1TB drive was only about 2/3 full as well, so there was plenty of space.  Funny thing was, after I did upgrade to SSD, I ran the Blackmagic Disk Speed Test again on the internal drive after formatting it to be used with Time Machine (formatted with Mac's Extended format) and was surprised I got more than double the speed ranging from 50-70MB/s and sometimes close to 100MB/s.  With those numbers I think a Mac with only a hard drive would be usable...but NOT WITH 25-30 MB/s.  Perhaps I could have done a clean install of the Mac OS and reinstalled all the apps instead of restoring from Time Machine and maybe that would have given me better numbers.  I don't know.It is a moot point now since I upgraded to an external SSD.  For the cheapest solution just upgrade using an external SSD with a USB 3.1 Gen 2 enclosure ($20-30) and use it on the Thunderbolt port, which supports USB 3.1 Gen 2 speeds.  Those speeds will beat the internal SATA 3 speeds.  If you really want to fully access an NVMe SSD's speed, then I suppose you can dish out the money for a Thunderbolt enclosure.  For me the new responsiveness of the iMac is worth it with USB 3.1.Aside from that this iMac is a great system with a great screen.  I can do a lot with the built-in 8GB RAM.  Don't let people tell you that is the bare minimum for MacOS.  The bottleneck won't be the RAM, it's the hard drive.  Also one good thing about this Mac is its upgradability.  There are lots of good instructions on removing the screen to access the CPU, RAM and drive, all of which you can upgrade.  Currently with my external SSD, I really don't see the need to do that...but if I need more power or RAM then at least I know it is possible.Pros:-You can find really good deals for this iMac, try to get one with the built-in discrete Radeon graphics not the Intel graphics-4K screen is awesome-nice expansion with Thunderbolt 3, USB 3.1 Gen 2, USB 3.0-Ethernet port-can boot to a faster external SSDCons:-hard drive can either be livable or too slow (you can try a clean install of the OS before upgrading to SSD)-to upgrade the internals you have to cut the screen from the adhesive, so be prepared to use new adhesive-there is a relatively large bezel around the screen, about 1 inch around.  Apple could have easily put a bigger screen in.