When staying in the 'zones', like aureolin - I think the payg oysters are great.
The only thing I would say with that, is to be careful how much you put on the card - too little and you will have problems, too much and it is money wasted, especially if you dont get down often.

To be honest, I have found the underground ideal to travel between Kings Cross and Euston (when needing a change on the Northern Line to Waterloo). It might be different if your not needing to stay underground though.

Re approach to Bank, Im not sure if it was because I wasnt expecting the drop, the darkness or the lack of driver. Havent experienced anything like it