Gotta say, I'm impressed. I've been tinkering with it for a short while, and already I'm happy. It's a complex beast, that's for sure, but there's a lot of convention (rather than configuration) that manages a lot of the complexity for you.
That said, there is little in the way of official guidance out there so far. Rob Conery's MVC Storefront screencast series has been a great resource for me, but sometimes I find it hard to follow. Meanwhile a lot of the really great blog posts are getting out of date as the framework matures.
Maturity is a good thing, though, even if it means the information we have on the framework is spotty. There are a few good books on the subject coming out soon, but that doesn't help me now. My employer is encouraging me to get acquainted with the framework, as we'll be using it (likely before the official release) to replace our customer-facing website. Which is exciting - I'm optimistic about the project - but also worrying, especially if there are large breaking changes between Preview 4 and release.
But alas. All this in the name of progress - and don't let my worries fool you, it is progress, away from the leaky abstractions and inane page lifecycle of Web Forms. It is not a step back as some profess. Certainly it is easy to use ASP.NET MVC to create code that is hard to understand - but the same is true of any technology, no matter the paradigm.
In short, I'm absolutely in love with the framework, and although I have my worries, I have no doubt I'll continue to use it in preference to Web Forms for all my projects moving forward.