Cons of IG Index

Bad Points

No Demo: No virtual 'play' trading account.
Spreads: They usually do not have the best share spreads but they make a market in more companies than the rest so sometimes I use them. For the bigger cap equity stuff there are definitely narrower spreads out there. Their main index spreads have however been tightened and now they are at least equal (if not better) to other providers in that area. For instance their spreads on the FTSE and Dax are now just 1 point (used to be two).
Limited Risk Account: May only let you have a controlled risk account (instead of the ordinary one), the controlled risk account comes with increased spreads. However, the limited risk (i.e. controlled risk) account isn't much good. Unless you are trading shares or commodities, you don't really need a guaranteed stop, which is a form of insurance (which you pay through the nose for). You can still use a GSO when required on a Standard Account. On a Limited Risk account, you must use it each time, which is bad for trading indices or currencies which are liquid enough. If you don't qualify for a Standard Account, try elsewhere...
Currency Exposure: With IG Index, you can have your account in Euros and trade stocks in countries which have the Euro (Ireland, Germany, Spain...etc) in Euro but you can't do that for others like UK, US stocks. What happens is that they take the deposit you need for the trade and convert it into pounds or dollars and therefore you have a currency risk for the period of the trade.
Platform Stability: Platform can be unstable during news times or times of high volume meaning you could get caught up in a negative wave without having access to your position.
Shorting Stocks Info: Even if a stock is listed on the IG site as being a acceptable SHORT it may not be accepted by IG. Also, they do not forewarn you if they are unable to accept any further bets on a particular stock (this is normal but its annoying to find out that you can't deal...). Happens usually with small cap but ruins your mental capacity before a trade.
More Transparency: I'd like more transparency up-front on each trade in terms of minimum/maximum bet size and minimum stop loss distance. I've had lots of trades rejected because I miscalculate these.
Margins: Looking at margin requirements, you need a lot more margin for IG trades (than for CMC for instance). It reduces if you place stops, but you still need to keep more money in the account to cover the trades. I take income, rather than upping my stakes, but have to leave quite a bit more in the IG every time.
Not for Scalpers: Reports of clients put on manual execution - spread widens on news releases. Screen updates can be slow when executing stops/entry orders. Hence when the market is moving rapidly you can be left 'praying' for a minute or more that your order/stop has been executed. In fairness I have never experienced non-execution of orders though slippage at volatile times has been quite wide. Not a problem for me though as I tend to trade at market rather than with orders. But they do say that as long as their quote touched that price then that's the price you get.
Some Slippage: You can get substantial slippage in a fast moving market, primarily because if between you clicking a price and it getting filled the price has moved, then it takes you to a new screen saying 'sorry but...', and by the time you get back to the trade screen you've missed out. Note though that slippage can also occur on a limit order whereby you would obtain a more favourable place so this cuts both ways really.
Guaranteed Stops Cost: The spread charge for using a guaranteed stop varies depending on what market it is one is betting on but isn't what one can refer to as 'cheap'. For example the guaranteed stop spread charge for betting on the daily FTSE is 4 points while the guaranteed stop spread charge for betting on GBP/USD is 18 points. Check the online dealing handbook to view all their spreads: http://www.igindex.co.uk/content/files/dealing_handbook.pdf
Daily Bets Expire on Daily Basis: Daily bets are opened and closed every day so profits and losses are crystallized at the end of the trading trading even if you let the position roll over. I consider this a disadvantage since it can create confusion as the entry level is changing every day. This also makes it harder to determine what the charges are for rolling the bets over. I download their trading log, but the rolls are inconsistent. Sometimes the position is closed and then re-opened, other times it's opened then closed, other days it will show 'Position rolled' - all for the same trade! I then need to calculate the price difference between the opening and closing prices to work out what the rollover cost actually is. Some other providers will not close rolling bets on a daily basis but will leave them open until the position is closed manually, this is better as it is more transparent and you can see the physical finance posting every day.
And of course IG like other spread betting providers have a disclaimer in its standard terms and conditions of website use to protect them against price discrepancies - so, it's down to you to watch out for this type of event that can seriously damage your pocketbook. Also, prices are usually slightly delayed but they insist that the graphs are indicative.