Love these.
The largest one I have is a king size night owl waffle one. Also have a double M&S one in another room. And some kids singles.
Pros-
I wash the entire duvet weekly, and I think the bedrooms smell a lot fresher for this (not just having the cover washed).
I always try to wash the bedding and have the same bedding back on at the end of the day rather than faffing about with second sets, storage etc. Making the bed at the end of the day is so much quicker and easier when it doesn't involve getting a duvet cover back on.
Much nicer to sleep with, without that feeling of your duvet moving about inside a cover.
Cons-
For king size, you do need a reasonable sized washing machine (mine is 14kg) and tumble drier, and be prepared to unload and reload in tumble dryer to move it around. That said I much prefer this to the faff with a duvet cover.
Tips:
Put the washing powder directly in the washing machine drum, not the drawer for good distribution. Give your machine an extra spin before tumble drying.
Wash and dry the duvet cover alone -better circulation of detergent/rinsing, better spinning and drying. And do the duvet cover first thing.
Don't (necessarily) bother getting winter weight versions. They are all lightweight by nature, to make them easily wash and dry-able. It's possible you might want a weightier blanket and bedspread on top if you are used to the feeling of something heavy to help you settle to sleep -you can just vary what you use depending on the time of year.