I've been with CMC Markets for a long time and my experience has been overall positive. I've also been a client of IG and City Index, also can't say anything negative about these two firms.
CMC market used to call themselves Deal4Free in old days, and were nicknamed Steal4Free by traders for the obvious reason. Probably why they changed names.
During price spikes these firms will have an excuse to fuck you over at the worst price. Trading is hard enough, you dont need your broker trying to steal from you account as well.