The top floor is called "the Living Room", apparently because the original design of the building envisaged its use as a residence for the Mayor of London. However, the first such mayor, Ken Livingstone, didn't like the idea; as a result, it's now hired out as a meeting space.
The views over the Thames are spectacular. Here's Tower Bridge, opening to let through a vessel with a smokestack made just the right length to force Tower Bridge to open when it goes through:
I wish the nearer tower didn't look alarmingly as if it were about to fall over from this angle.
Opposite City Hall is the Tower of London:
This is probably the best view you can get of it from anywhere, including Tower Bridge and the Shard. Ignore the glass, concrete and cranes and it's quite imposing.
Over to the left is the City:
I've no idea why the people who designed that awful walkie-talkie building didn't realise it was going to focus the sun and melt cars — it looks fairly obvious to me.
Sorry the pics are a bit grey, but it started to rain five minutes later. This is London, after all.