I'm rocking the 3.8 Ghz i5 with the 580 GPU and 1 TB SSD. I didn't bother adding any more RAM, figuring I should figure out how much more I need first. So far, I haven't noticed anything I need more RAM for. I'm doing word processing, family Photos, some desktop publishing and Photoshop work...and casual gaming.
My kids are Minecrafting on it right now. Minecraft of course doesn't challenge any modern machine, not even a Macbook Air. But it sure looks spectacular on the 27" screen. The kids are playing multiplayer: we have a gaming PC as well, a midrange Alienware machine. It doesn't have a tremendous GPU, but I was always irritated that it ran games better than my expensive Mac Pro cylinder. Well, no more envy. I sold that Mac Pro a month before Apple announced that a new one would be coming in 2018 (and also announced the iMac Pro). Recouped 85% of what I paid for it, held on for two more months, and bought this iMac. Feeling pretty pleased with myself for timing the market.
Our family likes Ark: Survival Evolved, which was horrible on MacOS on the Mac Pro, so bad that we had to use Bootcamp to get decent frame rates even on medium settings. No longer. We can bump it up to "high" (not "epic" yet) and everything is still smooth. (We do turn off the motion blur setting, which makes a difference.) I'm running the game in 2560x1440 and it looks unbelievable. Probably if I bumped it down to 1280x720, I could max out the settings. But I think it looks more spectacular at 1440, so I'll tolerate the lower framerate.
Also ran Grid 2, which my son and I like to play together. Just astonishing: highest settings, again at 2560x1440, and still over 70 fps. Bump down the resolution or the settings, and it jumps up over 120 fps. Of course, it has always been a super well-optimized game, and ran even faster on MacOS than on Windows, even on the Mac Pro.
I haven't noticed any problems with heat or fan noise. For sure it's not as quiet as my Mac Pro cylinder was, but the screen more than makes up for it.
I'll try to get some better info on frame rates once my kids get off the machine. 🙂