We reviewed it with Cvent, but decided not to use it. One big miss is they don't give you the ability to trigger actions based on the registrant type. For us, that's huge, because we have two separate audiences, and we only message to one via Marketo, so if someone from the other audience registers for an event, we don't want to sync them over to Marketo.
Also, I kinda wish the integration would've been more like the GTW or Webex integration so we could control the form, landing page, etc. I'm sure it's tough in terms of the payment collection process, though.