Not quite what I expected, but a trip highlight. There is an optional private tour add-on for Cardiff Castle: DO IT. We hadn't planned out doing that, but changed our mind at the last minute and WOW, was that a great decision. This tour does seem to depend a lot on your specific guide. There is a range of their fandom and knowledge base for the sites and how they relate to Doctor Who. The tour didn't match up with the exact description as listed in Viator, so I can't give it 5-stars, but it was a fantastic day that our family of 4 really enjoyed. Our guide was nice, fun and completely delighted our son. He also had a great recommendation for a lunch spot. The guide will help out those in the group who aren't Whovians with interesting things too.
PROS: You aren't dependent on public transportation to get you to Cardiff. The train wasn't running the day we went, so think of what a wrench that could throw in your plans. You get the iconic black cab experience. The add-on for Cardiff Castle is its own tour with a different guide who was a superfan and takes you where the general public can't go. It is a magical experience, and exciting for non-Whovians too. The 50 is for the group, not a per-person price, and totally worth it. There is some bonus Torchwood stuff in Cardiff as well. You get to see one of the few, still existing police boxes in London. You are picked up/dropped off directly from your hotel. Cardiff is a nice place to see, with a pretty harbor and lovely Pierhead.
CONS: The price tag is steep and basically just covers the ride to Cardiff and back a stop to see the police box in London. You pay extra for The Doctor Who Experience itself, food and the optional Cardiff Castle tour. We weren't shown any Doctor Who sites in London or, aside from the castle, in Cardiff.
OVERALL: This was a great day that the entire family enjoyed. Do not cheap out on the castle. You'll be thankful you took the tour.
TIPS: Bring umbrellas.