Great
Our tour guide was full of knowledge and personable. However, one MAJOR thing to note is that the "changing of the guard", the one at Buckingham Palace everyone thinks about, is only performed every other day, NOT daily. Our tour happened to fall on a day when there was no changing of the guard. Instead, we got the changing of the horse guard. And while this is cool in itself, it definitely is NOT what I thought I signed up for. It's not the tour operator's fault, or at least not the guide's, but several people in our group were very disappointed and felt deceived that the tour company made no mention of the every-other-day nature of the Buckingham Palace changing of the guard ceremony.
The tour of Buckingham Palace itself was excellent. It's open to the public only about 8 weeks a year while the Queen is away. The palace is beautiful and there was an interesting, well-presented exhibit on the Queen's garments from over the years. The audio tour of the palace was nice as well. Note that no photos may be taken while inside the palace.