While working on an app with lots of Windows Forms I've been thinking about the best approach for storing the data in a form.
On the web, since http is stateless, the form is implicitly separated from its state. You, or your framework, has to manage state; when a form is submitted its contents are sent from the client to the server, the server marshals this content into some data model, manipulates this data model, then rebuilds the state by creating an entirely new page. This balance of work shifts a little in the AJAX world. Instead of submitting the entire page to the server, the client submits asynchronous requests to the server, the server sends back results then the client updates the page (and, implicitly) the state of the app.
In client apps, such as a Windows Forms app, the form itself can store state. Since processing occurs in one place (the process running on the client's desktop), when the user changes which item is selected in a list this state is stored in the list widget. Despite this, my background from the web has resulted in a tendency to create separate data objects to store state then manually marshal data into and out of the form.
Having been "raised on the web" when Model-Controller-View was all the rage, I find this separation conceptually pleasing. UI changes were more frequent on the web than they are on the desktop so the separation dramatically reduced the amount of work involved in a page redesign. The web, being much more server-centric, also made heavy use of relational databases. Once you've worked out the relational data model then most of the work of designing the corresponding object model was already done.
In some cases the separation has helped. Having separate, non-visual data objects makes it easy to reuse these objects when an offline or batch mode app is needed. More generally, it makes reuse across apps easier since the data can be consumed directly without having to create phantom/hidden windows.
I also find the UI, even in client apps, to be more fluid than the data model. Sometimes it's easier to use radio buttons, sometimes it's easier to use a combobox. Sometimes, based on the user's choices, you want to swap out user controls. When the data model is separate from the UI, it's easier to experiment with different UI designs.
On the other hand, this separation comes with overhead. The overhead of both creating the data model and marshaling data into and out of the UI.
I don't know whether this cost is offset by the increased incidence of reuse and UI flexibility. Being able to reuse a section of data manipulation code, because it's manipulating data not the UI, does save time. Given a project with enough forms this time savings can be non-trivial.