I think I stayed here a few years ago on business, and don't remember being that impressed. But this time I had the family with me (wife and two boys aged 2 and 4), and I booked the Thistle because they had the family rooms with two double beds. And to my surprise, it was great.
Some other reviews commented that the rooms with two doubles weren't huge. Well, it wasn't a palace (the big place up the road was taken), but by central London standards, it was plenty big enough for our family, and had everything we needed. The rooms and public areas were well laid out, well decorated and immaculately clean (at least until our boys got stuck into the buffet breakfast - sorry about that). The staff were friendly and extremely helpful (even at that breakfast).
As others have commented, it's really well located if you're coming in from Gatwick (after a 12 hour flight with the boys, a hotel 5 minutes from the station was just what we needed). But better still, the open top sightseeing bus stops right outside the door. So two days' entertainment for the family sorted while I went off and did boring work stuff. Perfect.
The rooms are all on two floors. I didn't fully appreciate what a valuable selling point that is until the afternoon our two year-old son ran into the lift on his own. A scary moment, but thankfully it only took a couple of minutes to find him again. I'm glad we weren't in a bigger place.
I don't think we managed to get the cheapest rate available here, but even so we thought we got our money's worth (hey, it's London). I'd recommend it to others.
Summary
Customer Service
Quality of Room
Quality of Food
Cleanliness of Hotel
Value for Money
Family Friendly