Usually give free charting software and news feedPrices can be "smoother" and less volatile than ECN prices (this can be a con if you are scalping or trading very short term)Often have a more user-friendly trading and analysis interface
They may trade against you. In that case, there will be a conflict of interest between you and themThe price they offer you may be worse than what you could get on an ECNIt is possible that they may trigger stops or not let your trade reach your profit target levels by manipulating pricesDuring news, there will usually be a large amount of slippage; their systems may also lock up or not allow order placing during times of high volatilityMany of them discourage scalping and put scalpers on "manual execution" which means their orders may not get filled at the price they want.
You can usually get better bid/ask prices since they come from several sourcesVariable spreads between bid and ask may give no spread or tiny spreads at timesIf they are a true ECN, they will not be trading against you but will pass on your orders to a bank or another customer on the other end of the transaction.You will be able to offer a price between the bid and ask with a chance of it getting filledIf they support Stop-Limit orders, you can prevent slippage during news by making sure that your order either gets filled at the price you want or not at allPrices may be more volatile which will be better for scalping.
Many do not offer integrated chartingMany do not offer integrated newsMany of the trading platforms are less user-friendlyBecause of variable spreads (between bid and ask,) it may be more difficult to calculate stop loss and profit target in pips beforehand.Read the whole article:http://www.goforex.net/market-makers-ecns.htm