I went to the one in New York back in 2004 and the London one in 2009. At the time I genuinely enjoyed the New York one simply because I believed it was the dwelling place of "the governing body" - the holy of holies. Saw lots of printing machines and people being overworked with painted smiles. There was a little museum just to the right of the reception with CT Russell era stuff.
The one in London wasn't interesting at all. I remember they made us watch a video before the tour started. Cant remember what it was about but something along the lines of it's a privilege to be here. But I got what I could out of it. At one of the printing presses there were orange magazines being churned out and the guide told us not to look at what they're printing because it's a new mag for the convention (it turned out to be the "Origin of Life" brochure).

At the end of the London tour they took us to a room with thousands of books/literature in any language you could think of. People were taking random books as a souvenir.
Looking back, the people working there must have been overworked and stressed. The guy who's job it was to chop the convention badges into rectangles day in, day out looked like he'd died inside.