If you've never used a laptop before... it takes getting used to. Some people hate them because they don't feel the same. Others find that sitting in bed using a laptop is what they like best. I'm not a laptop person... don't like being constrained by their form-factor.
Also, going from a big screen to a tiny screen can be a bit jarring. I've never seen anyone wish their display was smaller. At work I have two displays that don't equal the one display I have at home. I'm constantly having to zoom in and scroll just to see what I am working on when I don't at home. It's exhausting sometimes.
Replacement for an iMac? No. These are low end models being introduced and are not meant to compete with the iMac. New iMacs with ARM will be coming over the next 2 years.
And boot time? At work my machine takes literally 10 minutes or more to boot... damn big brother software installed by my employer. We just changed owners... old owner, computer booted right up, new owner, takes forever. All because of software they added to keep an eye on me at work.
Edit: And one other thing, the new ARM Macs actually have less ports than their INTEL counter parts plus the RAM and likely the SSD are not upgradeable. If you have peripherals that need ports, you may find you don't have enough on these new devices.