Got mine last Friday and had some issues getting it set up because I decided on a clean install rather than a migration of all my old data. (Major housecleaning!) But that was my fault and I'm getting used to the reorganization.
Several days later now that the hard part is over I'm really enjoying using the new iMac. It replaces a 27" model from 2014 which I liked at first but eventually decided it was a desk hog and too big for my needs. With this new model it's easier to reach around back to connect a cable. (Still, I kinda wish there was one port on the edge for connecting things which are only temporary, like a backup drive or a phone.) The keyboard and trackpad are great. Having used a 12" MacBook keyboard and the one from my old 27" iMac for years, this keyboard has a much better feel. I haven't even unpacked the mouse yet. (Kinda wish I hadn't even bought the mouse now.) The system really flies. Apps launch instantly and I've seen no unexpected delays of any kind. The speakers sound fantastic from a close distance which is the only way I'll ever use them. I've yet to hear the fans come on. The Mac is almost too light because when connecting a cable to the back I nearly always unintentionally move it. But that's not a problem.
Just to comment on the naysayers; considering that lots of people, especially those on the go, do a lot of serious work on MacBooks with 12-16 inch screens, it's comical to say that 24" is too small. For the minuscule percentage who actually need giant displays, there are big options. As for some disliking the colors; who cares. I got green and I don't notice the color when using the computer. When the computer is off, the green looks very sharp and adds a little color to my otherwise dull room. My old gray iMac looks positively ancient and boring now. It'll be in a closet or on eBay soon.