I have not played with Soraka as a mid laner so far. Only as a support. And I think that currently she is one of the best champions for that role. I recommend activating her Astral Fusion ability first to be able to heal your teammates in the very beginning of the game.
Good luck and have fun.
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In fact, it's the best support I've seen so far. I started playing it with my friends in Custom games and damn, I'm going to switch my role and start playing support.