I'm honestly glad that the FBI is investigating this.
If it turns out that Clinton seriously mishandled her email account, then an indictment would force her out, clearing the way for a more electable Democrat.
If they find no evidence of wrongdoing, then this effectively becomes another Benghazi. (i.e. something that Republicans will go on about ad-nausium, with little effect since Clinton has an effective counter to the accusation.)
Imo, a win-win situation.
It depends on the timing. If, say, the FBI's report indicating wrongdoing came out in early January and it was incriminating enough to force Clinton out of the race, then Democrats would already have filing deadlines in about a third of the states passed and would have significant difficulty getting into the race. If it were done when 'tis done, then 'twere well/It were done quickly...
if something is really incriminating here, I hope the FBI reveals it by, say, Halloween, and we can move on. If it's much later, then Democrats will be stuck with the humiliating O'Malley/Webb/Sanders field going into the primaries.