I've been using City Index since Jul 07. I found their platform is not fast enough.
As for their customer service, I find that they have very helpful staff that helped me when there's a few hiccups. Like there was a night I tried to short EURUSD and it didn't execute. I didn't think too much about it, the next morning it actually went my direction! Then I tried to email them to ask about why it failed the night before. And my position was reinstated with instant profit.
I would rate their services as good but the platform as mediocre.