I've used theme too and gotten really great deal on spamalot and sound of music. Theatre tickets are much more expensive now then they used to be so any discount is good. They also have dinner and show together deals, which saves a lot of time and completes the evening nicely.

As I said in the introduction, we like London's theater very much. In the past, we used to buy our tickets at the TKTS booth, but with time, we started to get tired of the Q and not knowing for sure if we'd get the show you wanted. So we did some Googling and found out about http://www.broadwaybox.com/london, which offers nice discount codes on many shows, which you can use to buy tickets online well in advanced. At our last trip, we saw The Women in Black, which was great. We used broadwaybox code and got our tickets 2 weeks in advance at almost half the price, with very good seats. I think this site is a good bookmark for theater fans that are going to London or NYC.