I've been building a few sites with the framework since the first preview came out, and it has certainly come a long way already. It feels like a very light-weight and tidy framework.
There are a couple of areas where I think it really excels over "vanilla" asp.net:
- Enables a much cleaner separation of concerns/loose coupling
- makes test-driven development actually possible.
- And it's much more friendly towards javascript (ajax) heavy sites.
That said, there are some areas where it has some way to go yet:
- Validation
- Data binding
- Tag soup, as mentioned earlier (although this can be avoided to some extent; user controls, helper methods&codebehind is still allowed!)
The framework is still in beta though, so I expect these things to improve over time. Scott Hanselman has hinted that the Dynamic Data framework will be available for ASP.NET MVC at some point too, for example.