I pay (some £220/year) through work for the highest level of cover offered. In twenty one years, I've only ever used it twice, once for a broken limb and again for investigation of a potential eye issue which turned out to be nothing using more sensitive measurment devices.
DH, on the other hand, has more than got our money's worth over the years. He has tested the boundaries, too, so that we now know that anything considered chronic (ie not expected to get better) is excluded.
For acute conditions, our experience has absolutely been that the differences are in the over all service delivery rather than any difference in the level of medical expertise provided. Rather than be referred by a GP then wait for a hospital appointment, once referred (and with cover confirmed by the insurance company) a phone call to the specialist's medical secretary will be answered or returned in minutes, rather than hours. Appointments are flexed around any other business or private committments.
It's easy to see why it makes sense for a large company to provide this benefit, in order to minimise any disruption in the time off required. It does make me very loath to retire, I would not want to be without private medical insurance. I can no longer imagine having to just wait passively for an appointment rather than being aksed when would suit me to see the doctor. I also understand that the cost of cover will increase significantly with age so that it may become unaffordable even to pay myself. Geriatrics account for a majority of the NHS costs.
While I admire the NHS, I appreciate that people end up queueing because they do not have access to the more usual price mechanism. I would much prefer the concept of mandatory health insurance, usually provided though not for profit organisations, funded largely by employers with a public safety net for students and the unemployed.