IF, you have not tried it, don't worry, be patient, eventually we all get to lay down in one and go up in flames at the Mandai Crematorium.
In theory, a coffin is definitely claustrophobic. So logically if you chance to lay down in one while you're still alive, don't expect any comfort. It may be well padded (those high end models), but still, you're in a box - how comfy could that be??
You know, you can go Hong Kong ask the stars who starred in A Date with Vampire. They can tell you that.