Been a folding phone user for multiple generations now.
The flip-phone style ones: Excellent! The folding aspect really aids in switching between a small device with minimal controls to a normal phone. I would use these over a standard phone. No question.
The portrait-to-not style ones like pixel fold? Is nice on occasion but the software is a complete let down. For reasons I do not care: The orientation is either wrong or the software cannot handle the form factor. The end result is a poor user experience.
Considering "usable space" as a percentage of the screen that contains useful content these are worse when unfolded than folded. Yes the screen is larger but most of the screen ends up useless. The actual useful chunk suddenly becomes horribly small in proportion.
The default layout showing composing an email in gmail is a prime example. The actual content area of the email: Same size as when folded. The rest of the screen is unused or taken up by the keyboard. Yes the keyboard is now larger but it's somehow weirdly proportioned.
There are adjustments that can be made to improve that. Some have to be applied every time the phone is unfolded or the apps restart (or neither or both who knows?)
Oh and my pixel fold screen has failed 2x in 3 years - I do not treat it gently.
I go on a longer rant about this here https://youtu.be/WHxOosmJt_U