<p>Besides the unit testing benefits, what I heard regarding MVP pattern was that the reusability of the presentation layer.  So, you would design one presentation layer and use it for WinForms (rich) and Web.</p>

<p>I am currently working on a windows forms application in .NET with possibility of creating a web UI in the future.  However, when I am designing the presentation layer and the interaction between the UI layer, I am not certain whether this notion of reusability is worth all the trouble.  I sometimes feel like I am "dumbing down" my presentation for the possible web UI, when it can be so much more when designed specifically for the windows forms UI.</p>

<p>So, how many of you are reaping the benefits of the reusable presentation layer?
Does this reusability thing pan out in the real world?</p>

The value of MVC/MVP really lies in two different separations.
Separation between your presentation layer and your models (however you decide to implement them) is one of the more important principals software design for anything but simple systems. If you have any business logic or nonvisual logic in your application, you will definitely see the benefits of that separation.
The separation between presentation layer and controller seems a little less important to me. In rich client applications, you'll rarely see benefit from this separation; in web front ends, it's a lot more common (e.g., ASP.NET aspx and code-behind or J2EE jspx and servlet).
Maybe I don't completely grasp the way you've explained MVP, but I wouldn't say that one of the benefits is the
reusability
of the presentation layer--rather, I would say that the primary benefit is the reusability of the models. This is also one of the primary benefits of using any n-tier system. If you want to expand and build a new type of front-end (e.g., WinForms, WPF, etc.), you can do so without trying to separate all your logic out of the web application you just built.