It's definitely worthwhile going up there and there are good views to be had. We spent around an hour or so looking around, which seems to be a fairly typical length of time. I did not post any shots of the views as it was a dull day and they did not look particularly inspiring. The most interesting view was towards Tower Bridge and the Tower of London, but despite holding the lens against the glass I was unable to avoid reflections in that direction. I know that some people time their visit for dusk, when the lighting might be more inspiring if you are fortunate enough to get a decent sunset.
As you go in, you are forced to have your picture taken against a green screen and when you come out they have superimposed your image against a backdrop of the view. They are very keen to flog the pictures and there is a lot of shouting to get you take a look. I initially wanted to pose with my back to the camera, but was eventually persuaded to face it. Had I persisted, at least the ersatz image would have had me looking out towards the view.
The other thing to bear in mind is that there are no toilets up there. If you want to enjoy a few beers as well, it's best to do so afterwards.
Oh, and tripods are verboten.