Are most online ForEx brokers like FXCM or MB trading good at hitting your limit buy and sell orders? I've read so many mixed reviews online about slippage, scalping, etc. Many horror stories, but stories of success. If I have a plan that I've tested on a simulator that works, will it work with one of these brokers? (the simulator uses the same spread essentially with real historical data)

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I have traded both MB Trading and FXCM live and have not had significant liquidity issues with either. However, to answer your question as to whether "most online brokers" are good at "hitting your limit buy and sell orders" mainly depends on the position size your attempting to enter the market with, the time of day and currency pair your attempting to trade. And to answer your question whether your "simulator" will work with "one of these brokers" really depends on which code the simulator was created in. Is it an Expert Advisor coded for Metatrader? If so, MB Trading does not yet support the Metatrader software ( though are Rep there tells us they are working on it). MB uses the MBT Navigator which is much more difficult to create auto trading software for. FXCM recently began using Metatrader in conjunction with their FX Trading Station 2.0. Their Metatrader platform has huge drawbacks including not allowing demo accounts, not supporting multi-user accounts, and reportedly experiencing problems communicating between their FX Trading Station trade servers and their Metatrader Trade servers. I would recommend you google "Metatrader broker reviews" and find the brokers that best works for your situation.