>>>I want to create a tabbed UI. The controls that will appear on each tab should all be on a User Control which can then be added to the page at runtime.
>>>
>>>It's amazing that a "superior" tool like VS requires you to jump through hoops to do so many simple things.
>>>
>>>
>>Is it for Web application or Windows? I just did something similar for Web (have all tabs as UserControls), though I added them in design-time.
>
>WinForms

You can inherit UserControl in Win Forms, but I haven't played with it, only tried quickly when John Baird told us so.