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Canon Speedlites are just about the hardest thing to use properly in the whole system, a lot of users get caught out because they don't work well when just left on auto, they need user input and some knowledge, then they are just excellent
The advice to use M mode on the camera is so right, just select your SS, AP and ISO and use the FEC to control the flash brightness
The other thing they are really good at is fill-in, just set your ISO, use Av mode and select your aperture, then dial in as much or as litle flash as you need with the FEC, works perfectly.
Remember, there is no connection at all between exposure compensation and flash exposure compensation