Codebehind classes in a DLL separated from the ASPXs, good or bad?

[BH]

When adding a web form (or user control) in VS.NET, it automatically adds
the aspx, aspx.cs, aspx.cs.resx files. Several of the sample applications
I have reviewed recently including the ASP.NET Forum source code project
have the code-behind class in a separate class library project and compiled
into a different DLL. This DLL is then referenced in the web project and
the classes called in the corresponding aspx pages

e.g. <%@ Register TagPrefix="AspNetForums" Namespace="AspNetForums.Controls"
Assembly="AspNetForums" %>

While I can see this provides very nice seperation between the code logic
and the actual UI and allows different themes to be used against the same
set of code logic, I'm wondering if this makes the development process more
difficult. If anyone has developed projects in this approach and the
"default" framework provided by VS.NET (that is, aspx and aspx.cs are added
and linked together), I would like to hear some pros and cons.

Thanks for any comments
Bob