if by breaking in you mean someone smashing the windows, i don't think it is any harder.
ive heard somewhere that the doors are harder to jimmy open though than other cars, this might just be the newer models i have no idea.
it is also hard to get away with the bmws because of their starter disable, you have to have the key to start it.
but no matter what, get an m3, you won't be disapointed
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