The first (and, indeed, last) time I visited Heathrow's fifth terminal it was still being constructed, with only parts of the majestic terminal building (rather disappointingly called "T5A") completed.
As I recall, the little bus tour was more publicity exercise than construction site visit, but it was educational in its way, and fired my imagination and enthusiasm for the project. In particular I remember the so-called "angel trusses", used to support the massive roof - T5A is the largest free-standing building in the UK.
So it is that I find myself rather excited about my impending return to the "fully operational" terminal. I am to visit as a fare-paying passenger with British Airways; travelling to Sweden with the Sparkly One for the weekend.