Designers will work with MVC widgets the way they do with the previous web form widgets, via an Attribute that links it to a designer. You can build these with webforms (I believe MVC desigers might be supported too but I'll double check on the status), linking to the public properties of the controller just as you would with a standard web forms widget.
That being said, Thunder
is definitely the way to go, and I know for sure that the team is cranking the next release ASAP that will generate MVC widgets and their designers, so stay tuned!
The documentation for all of this is coming as well, there are already a few videos in the Getting Started Guide
hope this is helpful!