Writing for Macworld, Jason Snell gives it pretty good marks with a caveat:
The base storage configuration of the 4K iMac is a 1TB, 5400rpm hard drive. It’s been a few years since I regularly used a Mac with a spinning disk as its primary hard drive, and man, did it feel slow.
I agree with his suggestion: Don’t settle for a spinning drive anymore. Fusion or GTFO, which means this model starts at $1600. The non-Retina is still available with a speed bump starting at $1099 but with a spinning drive.
I am still rocking my 2007 2.4 GHz Core 2 Duo iMac as a media server thanks to having swapped in a hybrid drive. Yeah, it’s still slow, but the hybrid drive gave it another couple of years.
Jason also has reviews of the new Magic Keyboard and Magic Trackpad 2 on SixColors.