I visited Prague in Jan 2014 as part of a group of 6 travelers. The 6 of us did a 3 hour private tour of Prague Castle with Marketa. Overall, the tour was definitely a great experience. Marketa is a very knowledgeable and friendly tour guide and had lots to tell us on our 3 hour tour of the castle complex. Her English was great--we had no problems understanding her at any point during the tour.
Booking the tour was very quick and easy. Marketa was very quick to respond to emails throughout the booking process. She was also very flexible and willing to accommodate my travel group throughout both the booking process and the tour itself. She met us in our hotel lobby for the start of the tour.
At the end of the tour, Marketa suggested a restaurant near the Charles Bridge for lunch. She offered to take us there, and when we arrived at the restaurant, she spent a good 15 minutes giving us recommendations for restaurants, bars, and attractions and marking them on our map before she left. We really appreciated this, especially since I think our tour was technically over at that point. The food at the restaurant was very good and relatively cheap as well.
Overall, I think that our tour was excellent and of great value--the hourly rate for a 6 person private tour was very reasonable. If I ever come back to Prague, I would take another tour with Marketa. I would recommend Caput Regni and especially Marketa to anyone visiting Prague.