I have the LTD.
So far I have put about 3K miles on it. I am averaging 54mpg at normal highway speeds (almost all highway miles).
The seats are comfortable. Depending on what you like they may be more comforatble than the C-Max. Apple Car Play and Android Auto come standard. These are huge improvements over Ford and Microsoft early sync technology.
BlueLink is a feature offered on the car and free for the first 36 months. You can remotely start, stop, lock, unlock, etc. from your phone. No line of sight needed. This is perfect for hot days and you want a cool car or the opposite.
Handling is comperable to the C-max. It takes corners very well. The turning radius is much tighter.
I do like that it has a 6 speed transmission vs the CVT on the C-Max. You have a sport mode with quicker acceleration and the ability to shift manually (paddle shifting).
The rear seats do not fold completely flat. If you like the auto rear hatch this is not an option in the Ioniq. There is a little lift over to get in the trunk/boot area (with the seats up the area is larger than the C-Max).
The fuel gauge it doesn't show fuel remaining in gallons or how much was used. So if you are trying to squeeze the last miles in before fill up it makes it a little more difficult.
Overall I give it a huge thumbs up. This is the first car that I have found I would have changed from the C-Max for.