I was using CMC for quite a while and I found them to be ok. Not the kind of platform you would want to use for automated trading (i'm not actually sure if you can on their platform.) They run alot of get together meetings which were quite good for meeting people, but when I was using them, the biggest problem was thier massive java whale of a web interface.
I literally had to open another browser just to run CMC otherwise the java would run chrome into the ground.
Also, the guy who owns CMC used to be the treasurer for the conservative party in the UK, so I suspect if they ever get into trouble they will get bailed out.. =)