Most of my UI development lately has been in ASP.NET rather than WinForms. In ASP.NET I’m not a big fan of User Controls. The design time support is terrible, and since it is eventually on top of a document-centric HTML model, I find that I can produce more modular code by rolling my own UI classes.
In WinForms however, I have to say that User Controls are very nice. I’m using them for my blogging tool and I just love them. The design time support is marvelous. I just made a little HTML Editor control that wraps the DHTML control I mentioned earlier. By adding a toolbar and some HTML Tidy-ing, I can reuse it wherever I want.