One of the implementation strategies I used in the WPF app was to utilize the entity classes directly in the viewmodels. They come with all the necessary metadata for Entity Framework to know what to do with them, while the viewmodels simply leveraged directly from the entity's properties without exposing the entity directly to the views. The upshot of this is that it's a simple step from updating an object to saving said changes back to the database. There are no mappers required and there are no separate loads required to check for existence in the database. You're always in context and you're always in-memory. Of course, this app came with a local SQL Compact database, as opposed to running against a hosted API somewhere.
This doesn't work all that well on web, however. In fact, it doesn't work at all, since you loose context completely whenever a GET or POST request completes. So I've had to update the code significantly to deal with the difference in platform. I've had to introduce mappers (via Automapper) to deal with moving from entities to viewmodels and back. I've had to introduce a manager layer to better marshal calls into the repository, and of course the repository layer around EF has had to change significantly to first find an instance of the entity in the context before adding it if required during saves. All this now also comes with a custom IoC implementation (that doesn't hinge off a bootstrapper pattern).
Why am I doing this? Well, a WPF app locks me into the Windows front-end. It's possible to build that into an RT app or whatever for a phone or Surface, but most people don't want to install stuff anyway. If it's on the web, it's accessible, it scales and I have access to Google charts (although I was very well advanced in producing my own WPF charting library). If it's on the web, it's also easier to show it to people, so I figured it's better to combine the website and the app into a single entity.
I've now exceeded the feature set of the old WPF app, and I completed a first round of CSS and front-end work just last night, so there's still a long way to go. I'm not sure if I'm even going to try and monetize this, turn it out for public use or just keep it among friends. We'll see how that pans out. First and foremost, this is something I like to work on (which is a primary factor), and I don't want overhead or management that will detract from that joy. Secondly, I'm building this mostly for ourselves to use, instead of manually updating spreadsheets on a continuous basis. This being a web deployment now though, you have to have a certain level of finesse to it before sending the link to someone, because they can send the link to everyone!