See my ongoing review of the late 2015 iMac 5K.
All prior models of the iMac 5K have long been handicapped by a 32GB memory limit, thus forcing professionals to the Apple Mac Pro.
The late 2015 iMac 5K changes the game by accepting up to 64GB memory, which makes it far more viable for professional photographers, videographers, etc*.
My everyday photography work cannot be done efficiently on a system with only 32GB memory, as the screen shot shows from work just last night, where Photoshop alone uses 40GB of real memory.
Apple does not even offer a 64GB memory option, but OWC has a 64GB memory kit that costs only a little more than what Apple upcharges for 32GB, making the iMac 5K an awesomely fast Photoshop or Lightroom workstation at a far lower price than the Mac Pro. If a full 64GB is not needed, add memory to what shipped with the iMac 5K.
* Some iMac 5K drawbacks remain, name only two Thunderbolt ports (on one bus), which is problematic for multiple displays in particular, but also for multiple high performance devices, or device-to-device copies (bandwidth restrictions). Also, the Mac Pro can go to 128GB for really huge jobs.