I have an IG MT4 account. I've no complaints about it - rarely get rejections - but my system mostly places stop/limit orders rather than market trades.
However the spreads are wider and are very wide on indices overnight, margins are much worse too - but unfilled orders don't appear to use margin on the account which is very useful.
Position sizing is a little different too - the minimum sizes are larger and you can't go down to the penny level e.g. £2.01 bet size - it's generally at the 10p level.
The minimum stop/limit distances are wider - which will be a problem if you want to code up a short term trading system. I have had to code around this by programatically monitoring soft stops/limits when you need to move them to levels too close to the bid/offer for IG to accept.
The backtests (Strategy tester) are generally useless if you are using them over long timeframes as they don't simulate dividends and rolling charges which can take the sting out of your strategies.
So in summary - if you are an experienced coder - then yes IG MT4 can be useful - but if your knowledge is limited then perhaps not as there are a lot of limitations and differences to the Puredeal app.
Thanks for the detailed reply - it sounds interesting. I have a few follow up questions if you don't mind.
Is the backtest useful for intraday stuff then - trades that last 30 mins or so. Does the backtest use historical bid/ask or just work everything off the mid/tick price (which is my main issue with most backtests around)
Do you use a trade management library, and if so, which is best to use (I've spent a long time in the past coding my own so it'd be nice to use a ready made one)