It's... okay. Being able to TP to another lane that needs some patching up before backing is handy, being able to TP back to lane after backing for HP/items is also good.
I don't actually like taking Exhaust on Soraka because it requires her to be WAY closer to the fights than she ever should be, and even exhaust won't save her if she gets caught out of position.
She actually does pretty well with Clarity - her mana consumption is just far enough above her regen to make it so Clarity replaces a regen item, and since Soraka REALLY hurts for item slots with her hunger for AP, it's good to be able to free up a slot that would go to regen in favor of more utility or AP.
Of course, if you can persuade your ADC to not take heal, she does really well with that.