they are great.
They are super efficient. They are relatively cheap. and they are super easy to install DIY.
Plus they are super easy to retrofit into older homes that dont have space for duct work.
Plus Plus, while the unit can only be in heating or cooling mode (so no heating one room while cooling another), you can control temperatures and even on/off on an individual room basis so you can have your bedroom at a frosty 61f while keeping the livingroom at a more reasonable 74.