I have been a member for a year. What I like is that it is on the top floor of the County Hall with lots of natural light, unlike most gyms located in grotty basements. The swimming pool is very nice (25m x5m)and you can regularly find yourself swimming alone. But to me, it is often too cold (27 or 28 degrees C), I like it warmer than that. As there is only one studio, the choice of classes is limited (spinning, total body conditioning, pilates, yoga, step, dance...)but it suits me and I go to a different class everyday. What I really value is that there are max 8 to 10 people attending classes.
There is also a large room overlooking the London Eye with lots of machines (which I never use myself). So you can have you own program and get on with it.
The changing rooms have a sauna, steam room and jacuzzi, which have a tendency to break down regularly but are also fixed quickly. It is very quiet, especially at the weekends but can get a little busy with hens parties enjoying a spa day in the spring. Given the price (from £85 to £125/month), there are not so many young people. Most members work for companies located around Waterloo/Westminster or are SE1 residents (who enjoy a discount rate), hotels guests and the occasional celebrity. In summary, I like it because it is not busy and people don't take themselves too seriously.
The price is not cheap but there is no contract and you can suspend it. You can also use all the other Marriott hotels leisure centres in the UK, although I have never used any other one. Another advantage is you get 20% off their restaurants (I took family/friends for tea in their impressive restaurant/library overlooking Big Ben and the London Eye...).
When I joined last year, the rates were £110 for normal membership, £95 if you lived in SE1 and £85 for an off-peak membership. As I joined with my partner, we enjoyed a special price of £85/month each. I am not sure how the rates have increased since but it can't be so much. You can call them to have a look at the facilities. My advice: do not enquire directly through their website, it does not actually reach them.
Been a member for over a year and have found the staff to be genuinely friendly and a superb location, the facilities are impressive, and the hours are good. However there is an ongoing problem with things not working and often not all facilities are working at same time. More recently problems with the bathroom door handles and locker doors not functioning properly. Lockers Keys not having safety pins to attach to clothing give the impression of gradual detraction in standards. To top it all Prices have increased lately so I am going to be an ex member.