Thought I'd chime in since I'm dealing with the same issues. I've tried to integrate with TimeTrade but have hit some bumps along the ways. Here's my findings.
So if anyone hears of other viable integrations to book meetings, please speak up as I've been looking for one for a few months now.
What did you choose to go with? I am on the hunt at the moment for appointment tool that works with Marketo. I found LaunchPoint only had Box-Pilot.