I think $40 is a bargain price for better performance and security. Although, I haven't used a metro app that isn't a pile of steaming crap, that's ok. It might take some time for some metro style killer apps to be released.
I've used a handful of apps, mostly games, and they're pretty fun. I guess I'm easily amused.
My biggest complaint is that for apps like Wikipedia, they went out of their way to make it conform to Metro/Modern UI standards at the cost of convenience. The developers wanted Wikipedia to be a search provider for Windows 8, but searching with the search charm is the ONLY way to search in the Wikipedia. They couldn't be bothered with putting a textbox and search button anywhere in the interface.
Having the search charm work with the app is great, but who says that you can't include an additional search method as well?