Worth doing once for the experience if you are a "normal" person, or (as others have said) you are a celebrity avoiding getting hassled at the airport.

Not mentioned here, but AFAIU the genesis of this service/the Windsor suite, was that it was exclusive to and paid by the UK government for government business and offered as a "soft power" perk for visiting dignitaries - but they stopped paying for it exclusively a few years ago, and that's when BAA/HAL started offering it as a commercial service for all (that could afford it!). Maybe others can refine my recollection.