I will start the conversation and hopefully other customers will chime in because this is a hot topic at Imperva.
First, I think its important, at least in this forum, to define cloud. We have public clouds - AWS and Azure - and we have the Cloud WAF offering from Imperva.
For migration from an on-site deployment to a public cloud one, we believe a hybrid deployment is the way to begin. This way you can maintain your on-site deployment, while migrating to the cloud, while having a central management point for both. This is possible with our Flex licensing and offers the most flexibility for our customers. Your account team can discuss the options available with a hybrid deployment and see what is the best for you.
When migrating from on a on-site deployment to the Imperva cloud option it is recommended that you review your management options with your account team. The look and feel of the management capabilities are different as well as the options available. This is mainly so you will understand the situation going in and avoid any surprises once you have on-boarded. The protection offered is of the same caliber, and of course the cloud offering has a DDoS option.