Probably the best spread-betting firm anywhere.
*** UPDATE Nov 2015: Signs of deterioration. Support forum deserted, tech support sparse, software improvements patchy ( the best improvement I've seen in ages is showing p&l in pips on the Android app ), parts of website content 5 months out of date; important improvements not being even promised. No-one seems to care or know what is going on. ****
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I don't understand why some of these reviews are so bad. To me this is by far the best spreadbetting firm anywhere. I've been using them for a couple years now for day-trading ( many trades per day ), on a 5min chart. I trade only the S&P 500 DAILY and the the trade prices are perfect. I have a separate data feed for my other software and (bearing in mind the .15 extra spead) I have never seen a discrepancy between the CityIndex price and the global price, except that for some bizarre reason CityIndex ( and I assume many other SB firms) use a different rollover-date from the one agreed by the global market.
Anyway, the spead on the global e-mini S&P 500 is normally .25 and CityIndex add .15 to that, resulting in a spread of .4, which I think is fixed. So, the spread is probably the tightest in the industry and it seems to be fixed. I generally avoid trading around important news ( I don't think I can compete with speed of the computers designed to win with news trading), but I really doubt that splipage with CityIndex would be much worse than even with a DMA broker around the same location relative to the exchange. And there is a setting to limit the allowed slipage (but you may miss the trade if you don't allow any slippage in a fast moving market). During and after the news the price-candles are correct relative to the global prices from my other feed, so I really think the data is sound.
Their trading platforms are not bad, but thirteen clicks to place a bracketed order is way too many for me, so I very much prefer NinjaTrader for actual trading and finding set-ups.
They also publish an API, which I'm planning to use soon.
So, in short I would say that at least for the US SP 500 Daily Future, they are the best spread betting service by far.
PS. If someone at CI reads this, PLEASE get the damn rollover-dates for the front-month right.