I am a member (along with BoroughPaul !) of 37 Degrees at More. Never had any complaints so far since joining in January this year.
For the location near home, the price and the facilities, its a good deal. Go there about 4 times a week (either early morning, after work or weekends). There is a fairly good selection of popular classes - although I only do the Spin class, as I like loud and sweaty !!
They have some new staff as well now who seem to be making an effort to chat to members etc
Make them give you a guest pass first though so you can check it out when you would normally go - 6.00pm on a Monday or Tuesday is very busy !!
I am a weekend member at Fitness First London Bridge (Cottons). It's got a pool as well as a gym and, at £19.95 a month, seems to be good value. As a weekend member only, you can use it from 4pm on Friday afternoons, and all Saturday and Sunday. It may not suit everyone, but you do avoid the crowds and the gym is fairly empty.
If you go on the Fitness First website, you can request a free trial day, which may be worth a go.
I would not touch Fitness First with a barge pole. No complaint at all about the gym itself - I have gone to the one near St Paul's and it's excellent, especially the powerplate - but try leaving. They are an aggressive and ruthless outfit, who will harrass you for fees, threatening legal action until you give in. There are hundreds of complaints about FF on the web. Just google a few and you will see what I'm talking about. If you think you've signed up for, say, three months, think again. They are masters of the fine print. If you want to stay with them for life, fine. But be warned.