I put this iMac into service February, 2018.  Even though it was refurbished, it seemed to be in new condition.  Everything works and it is much faster than other Macs I have (due to Quad core).  The screen is bright and sharp.  The Magic Mouse is great.  I wish the keyboard had a ten key number pad.  I've been back with Mac since 2004 after living in the PC world for almost 20 years.  Neither world is perfect, but I enjoy the integration of the Apple eco-system and use it to the fullest with iPhone, iPads, and AppleTV's.  The way that family works together is without parallel in the PC world.========Update (2 years later): Ultimately I found the internal 1TB HDD to be too slow...bootup took forever...so I took it to my local Apple certified support store and had them swap that out for a 256Gb SSD.  It now boots very quickly and seems to have less problems with the OS clean-installed on that SSD.  I keep all data on an external Drobo drive so that all that is on the SSD permanently is the OS...which leaves about 185Gb of temp storage space for working space.  It also works well for iCloud documents which disappear from the local drive after a period of inactivity with a given document.==========Update 2 (after 4 years)I have four Macs in operation ranging in dates from 2010 (running El Capitan) to a new M1 MacMini.  I do have Big Sur running on this 2015 iMac without problems or stalling issues.  Of the four, I think this iMac may be the most dependable and robust of the lot.  It is really a good computer (after the SSD upgrade I mentioned above)....maybe the best Mac I've ever owned (and I'm sure I've had at least 10).  This is a heqq of a value.