I don't think we've learned anything yet, so I can understand the frustration of Clinton and the Democrats. However, I feel like Comey was between a rock and a hard place. He released this info which starts a firestorm of complaints saying it will sway the election in Trump's favor. If he didn't then post-election there would be a firestorm of complaints by Republicans that he didn't release it in order to help get Clinton elected.
Comey doesn't report to Congress, ordinarily. Congress can subpoena him, or they can ask him for information. Taking the step of writing to Congress is unorthodox, and it should not be done without some kind of actual relevant information. He should have conducted his investigation, and released a statement when they have something to show for it, if ever.