What is Microsoft Flow?
Microsoft Flow is a cloud-based service that makes it practical and simple for line-of-business users to build workflows that automate time-consuming business tasks and processes across applications and services. It is a separate product from SharePoint, but it already has very good integration with SharePoint and Microsoft OneDrive.
Here is the Microsoft Flow basics webinar video:Here are the presentation slides
Is Microsoft Flow useful for SharePoint Developers, DevOps or Power users?
The intended audience for Microsoft Flow is line-of-business users (citizen integrators, power users) for now ... I see very powerful features for developers already introduced so for me it could be used as developer tool in some scenarios. Do not think of it as the one tool to rule all your SharePoint intranet automation because has its limitations.
Will Microsoft Flow replace SharePoint Workflows?
Yes. Microsoft are pushing in that direction, it has easy to use drag and drop designer (in browser), it has better integration with external SaaS vendors. If you are about start automation and need to decide on SharePoint Workflow or Microsoft Flow better have a look at Microsoft Flow. Of course the SharePoint Workflow is out there for ages so it has some functionalities still not available for Flow so it depends on the particular business scenario and the use of the right tool to do the job.
Microsoft Flow better SharePoint fields support announced recently
The webinar video specifies that write to some SharePoint field types is not available, but few days after the webinar the Flow team announced write support for them: Setting SharePoint’s Person, Choice, and Lookup fields. This how quickly the cloud moves :).
Consider Azure Logic Apps for advanced development scenarios
Microsoft Flow and Azure Logic Apps share the same platform. Logic Apps is an Azure service that provides the same great features of Microsoft Flow, plus features such as integration with Azure Resource Manager and the Azure Portal, PowerShell and xPlat CLI, Visual Studio, and additional connectors. So for more complex business cases Azure Logic Apps may be the right tool (service).
Microsoft Flow is great service that can be used to do the right job and save you time so do not hesitate to use it. I remember the time when we did not have such automation serverless services and we have to automate and integrate with months of efforts in custom development, but now we can do the same with few drag and drop moves in the browser.