Hello Chris,
We are currently using CVent but are not happy with the results and have plans to drop it at the end of our annual contract. The main issue we faced was the sync between SFDC and Cvent, while this is not a direct issue with Marketo, due to SFDC syncing to Marketo and vice-versa it became a Marketo issue by default. We were never able to get the synch to work with any degree of accuracy. Standard fields such as; city, state or email would oddly/randomly not sync with SFDC. We even had a consultant from Cvent in for a visit, and he admitted that there were some software issues that can break the sync.
It's very possible that by removing SFDC from the process that Cvent and Marketo play well together.
We plan on using a combination of AtEvent, WordPress and some custom coding to replace Cvent.
-Matt