For me, location is the most important thing, and I tend to go with whatever is closest to my flat, so that it's easy to get there (no excuses!). I should also note that I don't tend to use the showers or change at the gym, so I don't get too concerned about those facilities.
I've belonged to 3 different gyms since I've lived in London.
The first gym I joined was our council's leisure centre. I thought that this would be cheaper than the chain gyms, and that I would use the large indoor pool. The facilities weren't that great, to be honest, and in doing further research I realized that the rates were actually on par with the chain gyms. I also never used the pool. I definitely wouldn't discount your council's leisure centre, as I know that some are fantastic, and people have been really pleased with them. It just didn't work for me. Also, outside of London, rates are probably considerably cheaper.
The second gym I joined was Virgin, and I was happy with it. It did get a bit busy during peak hours, and sometimes I couldn't get on a machine I wanted. Facilities and staff were good. Generally satisfied with it.
I now belong to Fitness First, and I really love it. It's a smaller branch, and everyone there is really friendly. I really like the teachers/trainers there, and have gotten into a lot of classes that I haven't tried in the past. Nothing too terribly special about the facilities, I just really like the people/classes.
I will offer the caveat that among the chains, things can really vary from location to location. Also, fees vary from location to location. I looked into joining the local Virgin when we moved, but the fees were much higher than we paid in our old location (no pool at the old one), and much higher than Fitness First (which generally tends to be on the cheaper side among the chains, I believe).
