I frequently get questions regarding the use of usercontrols with in SharePoint WebParts. Someone often has a usercontrol they'd like to reuse inside of SharePoint. Sometimes developers like being able to use the designer views in Visual Studio when developing WebParts. It is possible to display a user control from with in a SharePoint WebPart. I've prodived the following sample Visial Studio Project (C#) that shows how to do this. This is a quick and dirty example, and is by no means an official recommendation.
I have attached the sample project to this blog post. (View the blog posting individually to see the attachment download link)