Yes, I am late to the party, but now that the Apple watch has completely destroyed the atmosphere at the Apple Regent Street store in London and left the place feeling a bit "tombish" I finally had a chance to actually spend some time playing around with the 5K iMac at the store. (usually there is somebody doing their facebook or whatever for a few hours and it's impossible to actually try it out) The lag in mission control is shocking. I opened a few apps, moved them into different spaces and then flipped between them. I also hold shift when going into mission which slows the animations down so you can really see how smooth (or not) they are. I was shocked that the R9 M290X cannot handle basic animations and everything is very jumpy and laggy. Perhaps the display model has not been updated, but I hope that my experience does not match the real world, or that the M295X makes things a bit more bearable. My 2012 rMBP is much smoother when subjected to the same test and it is hardly cutting edge considering how old the internals are.